[mjn]

Just new enough that you can usually safely dig without too many delays, though. A problem with tunnel construction in very old cities, in places like Italy and Greece, is that you have a high probability of running into an interesting archaeological site, and then you have to pause construction and decide what to do about it. E.g.: http://www.bbc.co.uk/news/world-europe-21743758

[alister]

> in very old cities ... running into an interesting archaeological site

Why are archaeological sites are so far underground? Are ancient cities actually getting higher in altitude as old ruins get covered up?

Why Pompeii was buried is clear. I am more curious about continuously occupied cities like Rome. How did the ruins get underground there?

The best answer I could find was this:

http://www.straightdope.com/columns/read/813/how-come-archae...

It says that abandoned ancient cities have in fact risen in altitude over the centuries becoming "mounds". But it also says that Rome's famous ruins are at surface level, but doesn't explain ruins that have been found deep underground. So I'm left wondering if Rome and cities like it have gotten a lot higher or not.

[tallanvor]

Well, it's not so much that the ruins are likely to be found "deep" underground, but some are as much as 3 or 4 stories underground. It used to be that it was easier to build on top of existing structures - or the remains thereof - rather than cart the trash out and start fresh.

If you ever go to Rome, you'll notice that the Colosseum, for example, is at surface level, but most of the land around it is raised up. During street work, you can see parts where they've uncovered foundations, often digging down quite a ways to get to what was the street level at the time. In a place like Rome, when they uncover new ruins, they'll generally be studied for a while and then covered back up because it's the easiest way to preserve the site because they still need the land above it to be usable.

[lnanek2]

Don't forget that things aren't the same everywhere. The land in Alaska and New York is rising pretty fast, geologically speaking, since the weight of glaciers came off. Meanwhile New Orleans is sinking. Chicago took matters into their own hands and raised street level an entire floor to combat flooding. We still have ancient caves at ground level in New York, but I'm sure stuff buried under a city survives better.

[Piskvorrr]

"raised street level an entire floor to combat flooding" - yup, that's pretty much the same technique by which "the ancient cities are rising." Except that Chicago was lifted up, whereas the old cities were only readjusted - essentially, ground level becomes the first basement, entrance is now on what used to be second floor. This process could be repeated, giving rise to old ground-level floors becoming the third basement or similar. Case in point: Prague's Old Town, founded at riverside, was gradually raised by up to 30 feet (which has unexpected consequences, and actually worsens the flooding problems it was always expected to solve).

[filpen]

> Are ancient cities actually getting higher in altitude as old ruins get covered up?

I recently watched a TV show from a relatively old series about underground cities where they showed the underground of Istanbul. Very interesting, but I can't remember the name of the show.

From http://en.wikipedia.org/wiki/Hippodrome_of_Constantinople for instance:

"The course of the old racetrack has been indicated with paving, although the actual track is some 2 m (6.6 ft) below the present surface. The surviving monuments of the Spina (the middle barrier of the racecourse), the two obelisks and the Serpentine Column, now sit in holes in a landscaped garden."

[swombat]

That has happened with Crossrail actually... for example, the Liverpool Street station digging uncovered countless artefacts from Bedlam, aka Bethlehem Mental Hospital. Human remains and other things that can't just be tunnelled through = delays...

[trumbitta2]

Not so new as you may have thought: http://en.wikipedia.org/wiki/Londinium

It's just way bigger than its ancient self, so you are less likely to find archeological sites.

[lclarkmichalek]

That happened quite recently in London for the new CrossRail track: http://www.bbc.co.uk/news/uk-england-london-23609994. Not that I agree that the city is too old, or that this is a major problem; from the sounds of it, the CrossRail planners were well aware they'd run into interesting archeological finds, so allotted resources to deal with it.

[ido]

Infrastructure work in Jerusalem supposedly often runs into such problems too.