Daniel sent us this one — and I love this because it's exactly the kind of thing where you nod along in conversation and then realize you have no idea what you're nodding about. His wife Hannah, who's an architect, throws around the word "bedrock" when she's talking about construction stages, and Daniel had the honesty to admit he doesn't actually know what that means. Is it a specific depth? A specific kind of rock? Or is it just a hand-wave for "we stopped digging"?
He's asking specifically about Jerusalem — what's under our feet here, how deep the really hard stuff is, especially now that high-rises are digging deeper than they ever have before.
And this isn't just geology trivia. Jerusalem is in the middle of a genuine high-rise boom. Towers are going up everywhere, foundations are going deeper, and whether a building stands or settles — literally whether it stays where you put it — comes down to what the engineers hit when they stop digging.
The word "bedrock" does an enormous amount of heavy lifting in casual conversation. People use it to mean "the solid thing underneath everything else." But on a construction site, it's a very specific engineering threshold. It's not a single layer, it's not a fixed depth, and it's definitely not the same rock type everywhere you go.
Which is exactly where Daniel's confusion comes from, I think. You hear "we hit bedrock" and you picture this clean, universal floor of granite sitting down there like the basement of the planet. And that's not what's happening at all.
Not even close. What's actually under Jerusalem is this layered sequence of limestone and dolomite that was laid down about a hundred to ninety million years ago, during what geologists call the Cenomanian-Turonian period. And the key thing — the thing that makes this whole question interesting — is that some of those layers are soft enough to carve with hand tools, and some of them are hard enough that you need hydraulic hammers just to chip away at them. They're all "rock" in the everyday sense, but only some of them count as "bedrock" for engineering purposes.
The short answer to Daniel's question — "how deep is the really hard stuff?" — is that it depends entirely on where you're standing. In the Old City, you might hit it at the surface. In the valleys, you could be digging thirty meters down before you get there.
That variability over very short distances is one of the things that makes building in Jerusalem uniquely complicated. You can have two tower projects two blocks apart, and one hits competent bedrock at five meters while the other is still breaking through soft limestone at twenty-eight.
Which also means that when you're living next to one of these excavation sites and you hear that menacing rumble — the one Daniel mentioned in his prompt — the sound itself is telling you something about what layer they're in. The pitch of the hammer changes depending on whether it's chewing through the soft stuff or pounding against the hard stuff.
And that's what we should dig into — literally. Let's walk through what's actually under the ground here, from the surface down.
The first thing to understand — and this is where the word "bedrock" really falls apart — is that from an engineer's perspective, bedrock isn't a thing you identify by looking at it. It's a strength threshold. They care about something called bearing capacity, which is just how much load the ground can support before it fails, measured in megapascals or tons per square foot.
You could have two different kinds of rock, and one counts as bedrock for a five-story building but not for a thirty-story tower.
The number matters more than the name. For a thirty-story high-rise, you typically need bearing capacity in the range of maybe three to five megapascals minimum — that's roughly three hundred to five hundred tons per square meter. And Jerusalem's geology gives you layers that range from basically zero in the fill and soil, up to eighty megapascals in the really hard dolomite. That's an enormous spread.
Which means "we hit bedrock" is a statement about hitting something strong enough for what you're building — not about reaching some universal floor of the earth.
And in Jerusalem specifically, what you're digging through from the surface down is this sequence. First, there's the artificial fill — three thousand years of collapsed buildings, pottery, rubble, debris. In the ancient city core that can be ten to fifteen meters thick before you even touch natural ground. Then you hit the Terra Rossa soil and weathered limestone, this reddish marly layer, usually two to five meters. Below that is the Meleke limestone — and this is the twist. Meleke is rock. It's the stone they quarried to build the Temple Mount retaining walls. But it's soft, only about fifteen to twenty-five megapascals. For a high-rise, that's not bedrock.
The ancient builders were essentially carving their building blocks out of the stuff that modern engineers would reject as too weak for foundations.
Which is a beautiful irony. The stone that built Jerusalem's most famous structures isn't strong enough to support a modern tower. You have to go deeper, through the Meleke, until you hit what the local quarrymen call Mizzi Hilu — that's a hard dolomite, fifty to eighty megapascals. That's your competent bedrock. And depending on whether you're on a hilltop or in a valley, that layer could be at the surface or thirty meters down.
Let's make this concrete. If you're standing on the Temple Mount, or up on Mount Scopus, the Mizzi Hilu dolomite is practically at your feet — maybe a meter or two of fill and weathered stuff, and you're there. But if you walk down into the Tyropoeon Valley, that same layer drops twenty to thirty meters below street level.
That's the topography doing the work. Jerusalem sits on a series of ridges and valleys — the Judean Hills — and the bedrock surface follows that ancient topography. The valleys filled in over millennia with eroded material, debris, all the stuff that washed down from the hills. So the depth to competent rock isn't random, it's basically a mirror of the landscape before humans ever showed up.
Which means a developer on a hilltop gets a bargain on foundation costs, and a developer two blocks downhill is suddenly staring at an extra twenty meters of excavation.
That's exactly what we've seen with the recent high-rise projects. The Holyland Tower complex — they had to go to twenty-eight meters before they hit competent Mizzi Hilu dolomite. The Mamilla project reached it at about twenty-five meters. The new Knesset Tower, which started excavation last year, reportedly hit the same layer at twenty-eight meters.
Twenty-eight meters. That's roughly the height of a nine-story building, just to get to the point where you can start building upward.
It's expensive. Every meter of excavation in urban Jerusalem is slow, because you're not just digging dirt — you're going through that archaeological fill layer first, and Israeli law requires a salvage excavation. Archaeologists have to document and recover everything before the engineers can proceed. That's why some of these projects add six to eighteen months to the schedule before a foundation is even poured.
When you hear the hammer going for months on end, part of what you're hearing is the archaeologists' schedule, not just the engineers'.
And once they clear the fill and the Terra Rossa, they hit the Meleke limestone — and this is where the noise really picks up. Meleke is soft enough that the ancients quarried it with hand tools, but for a modern excavator it's still rock. Hydraulic breaker hammers pound through it, and that's the deep, percussive rumble people feel in their apartments.
Here's the thing I want to nail down — you said Meleke is fifteen to twenty-five megapascals. That's still stone. You could build a five-story building on that, couldn't you?
For low-rise construction, Meleke is perfectly competent bedrock. The problem is when you're putting up a thirty-story tower. The load concentration at the base of a building that tall is enormous. Fifteen megapascals might hold it, but you'd be right at the edge of the safety margin, and differential settlement becomes a real risk — one corner of the building settling a few millimeters more than the other, and suddenly you've got cracks propagating through the structure.
"competent" is a sliding scale. Meleke is competent for some things, not for others. Mizzi Hilu is competent for pretty much anything you'd build.
The numbers make this vivid. Mizzi Hilu dolomite — fifty to eighty megapascals unconfined compressive strength. That's roughly five thousand to eight thousand tons per square meter. For a thirty-story tower, you need maybe a tenth of that. So when engineers found on Mizzi Hilu, they're not just meeting the requirement — they're getting a ten-to-twenty-times safety factor baked into the ground itself.
Which is why they keep hammering through the Meleke until they hit it. Nobody wants to be the engineer who said "eh, this is probably fine" about the softer layer.
There's actually a sharp contact between the two — you can go from twenty megapascal rock to seventy megapascal rock in a matter of centimeters. The hammer operator knows it the moment it happens. The pitch changes, the resistance changes, the whole rhythm of the work shifts.
That moment — that change in sound — is exactly what Daniel's prompt was circling around. The difference between "we're still breaking through the soft stuff" and "we've hit the real floor.
There's one more layer below Mizzi Hilu, by the way — the Mizzi Ahmar. Even harder dolomite, notoriously difficult to excavate. Most projects never reach it because Mizzi Hilu is already overkill for anything being built. But as towers go deeper for parking and sub-basements, some of the planned forty-meter-plus excavations might start brushing up against it.
The really hard stuff has an even harder cousin waiting underneath.
Jerusalem doesn't make anything simple.
That sharp contact between Meleke and Mizzi Hilu — that's where the real engineering drama lives. If you misidentify the layer, if you stop your piles in Meleke thinking you've hit the competent stuff, you've just built a thirty-story tower on rock that's only a quarter as strong as what you actually needed.
What does that failure actually look like? The building doesn't just tip over.
No, it's subtler and slower. You get differential settlement — one corner of the foundation settles a few millimeters more than the rest. Over months or years, that tiny difference propagates upward through the structure. Cracks appear in the shear walls. Doors stop closing. The building doesn't collapse, but it becomes a permanent headache, and fixing it is astronomically expensive.
There was that parking garage collapse here in two thousand seven — was that a foundation issue?
That one was structural, not geotechnical — a failure in the concrete frame, not the ground underneath. But it's the same kind of lesson. When you cut corners on what's holding the thing up, the bill comes due eventually. With foundations, the bill arrives in millimeters per year.
Which makes the hammer operator's ear more important than most people realize. The sound shift when you cross from Meleke into Mizzi Hilu — that's not just interesting geology, it's a safety check.
It's audible from outside the site. If you're Daniel, living next to one of these excavations, the day the hammer pitch drops and the percussion tightens up — that's the day they hit the real floor. You can track the project's progress by ear.
Which brings us to the other thing that makes Jerusalem excavations sound different from anywhere else — the archaeology. You mentioned the salvage requirement earlier. What does that actually do to the timeline?
It's the wildcard. Israeli law mandates that any construction in an area of archaeological sensitivity — which is basically the entire city — has to be preceded by a salvage excavation. The Israel Antiquities Authority sends a team, and they dig through the fill layer by hand, documenting everything. And in Jerusalem, that fill is not just dirt. It's three thousand years of collapsed walls, broken pottery, coins, mosaics, sometimes entire buried streets.
Before the hydraulic hammer even shows up, there are archaeologists with trowels.
Sometimes a year and a half. The Western Wall Tunnels excavation went through fourteen meters of fill before they reached the Herodian street level — and that was all archaeological material. Every centimeter had to be documented. For a modern high-rise, you're looking at six to eighteen months of archaeology before the engineers can even start thinking about foundations.
Which means when you're living next to a site and you hear nothing — no heavy machinery, just quiet scraping — that's not a delay. That's the archaeologists doing their legally required work.
It's some of the most productive archaeology in the world. Construction projects in Jerusalem have turned up everything from First Temple period seals to Roman drainage channels to Crusader marketplaces. The fill layer is essentially a compressed museum of the city's entire history.
The excavation is loud in two completely different ways. The archaeology phase is quiet but tense — every scrape might hit something priceless. Then the hammering phase is deafening but predictable — they're just eating through the Meleke.
The contrast with other cities is striking. Take Manhattan — the bedrock there is schist, and in Midtown it's practically at the surface. You can see it outcropping in Central Park. But go downtown to Battery Park, and that same schist drops to thirty, fifty meters deep. The skyline of Manhattan is literally shaped by where the bedrock is shallow enough to build cheaply.
The skyscrapers cluster where the rock is close.
There's a famous paper about this — the geological reason the Financial District and Midtown have the tallest buildings while the area in between is lower. It's not zoning, it's schist depth.
London is the opposite extreme.
London sits on the London Clay, which goes down over a hundred meters before you hit the chalk bedrock. So London's tall buildings rely on deep piles friction-gripping into the clay rather than sitting on hard rock. Completely different engineering approach.
Jerusalem is neither of those patterns. It's not a consistent depth that changes gradually over kilometers, and it's not a single deep soft layer. It's this jagged, unpredictable sequence where the depth can change by twenty meters in the space of two blocks, because the bedrock surface is essentially a buried landscape of ancient hills and valleys.
That's what makes the local knowledge so critical. You can't just pull a number from a city-wide geological map and assume it applies to your lot. You have to do site-specific boreholes. Every single project has to discover its own stratigraphy.
Which also means that when a developer tells you they "hit bedrock," the only responsible follow-up question is: which bedrock? And at what depth? And what was the bearing capacity?
If they look confused by the question, that's your answer right there.
All of this adds up to three things you can actually use. And the first one is exactly what you just said — when someone on a construction site tells you they hit bedrock, the only smart response is: which one? Meleke or Mizzi Hilu?
Because the answer tells you whether they're done with the noisy part or just getting started on foundations. If they say Meleke, the hammer is still coming.
Second — and this is for anyone buying an apartment in one of these new towers — ask the developer what depth they excavated to and what rock type they founded on. If the building is over ten stories and the answer is Meleke, that's a red flag. Meleke at fifteen to twenty-five megapascals might hold, but you're right at the edge of the safety margin.
You don't want to be the person who bought on the fifteenth floor of a building where the engineer decided "probably fine" was good enough. Ask for the bearing capacity number. A developer who knows their project will have it. One who doesn't — that's your cue.
Third — the rumble Daniel mentioned, that menacing sound coming up through the floor, is actually the sound of progress. It's the hydraulic hammer chewing through Meleke limestone. When the pitch suddenly changes, or the hammer goes quiet, they've either hit the Mizzi Hilu dolomite — which means foundations are next — or they're switching to drilling and blasting for deeper work. Either way, the noise is the dig proceeding exactly as designed.
It's the opposite of a warning sign. It's the sound of the building getting its feet under it.
Which raises the obvious question about what comes next. Some of the towers on the drawing board are planning forty-meter-plus excavations — deeper parking, deeper sub-basements. And below the Mizzi Hilu sits the Mizzi Ahmar, which is an even harder dolomite layer. The local quarrymen gave it that name for a reason — "ahmar" means red, and it's notoriously stubborn to excavate. The compressive strength on that stuff pushes past a hundred megapascals in some samples.
The hammer that's already shaking your apartment might get an even angrier cousin.
It's not just the noise. The cost curve on excavation isn't linear — when you go from breaking fifty-megapascal rock to hundred-megapascal rock, the equipment changes, the pace slows, the budget inflates. Developers are going to have to decide whether the extra parking levels are worth the price of chewing through Mizzi Ahmar.
Depth isn't the only thing that changes when you dig deeper. You mentioned the water table earlier.
Right — and this might actually be the bigger story going forward. Jerusalem's water table has been rising. The mountain aquifer that sits under the Judean Hills has been recovering because Israel reduced pumping from it over the past couple decades, shifting more to desalination. So the groundwater is higher than it was thirty years ago, and it's still climbing.
Which means a forty-meter excavation isn't just a rock problem. It's a waterproofing problem.
You're putting a basement four or five stories underground into rock that's increasingly saturated. And dolomite — even the hard stuff — is fractured. Water moves through the cracks. So now you're not just supporting the building, you're keeping a permanently dry space inside a rock formation that wants to weep groundwater.
The next generation of Jerusalem towers has to solve for harder rock and wetter rock at the same time.
That's the open question I keep coming back to. We've been talking about what's under the ground today. But what's under the ground is changing — slowly, but in ways that will shape what gets built and what it costs. The Mizzi Ahmar is down there waiting, and the water table is rising toward it.
That's a good place to land. And now: Hilbert's daily fun fact.
Hilbert: In the late sixteen hundreds, astronomers observing from the Outer Hebrides documented that lunar libration in latitude reveals roughly forty-one percent of the moon's surface over time — meaning from that single windswept archipelago, they could map nearly half of a world they'd never set foot on.
Forty-one percent from the Hebrides.
This has been My Weird Prompts. Thanks to our producer Hilbert Flumingtop. If you want more episodes, find us at my weird prompts dot com. We'll be back soon.
