> When most cities build metro systems, they simply blast through rock.

I'm not sure that this is true (ignoring the bit about blasting, which hasn't been a standard way to build metro lines for a very long time, of course); there was a lot found when London's Crossrail was built, say.

(While it's not made totally clear, I assume they're mostly finding stuff building stations, not tunnels. The tunnels are ~20m below ground, but the stations have to go all the way to the surface.)

Really, any old city, anytime you build anything you may find something. For instance, here's a supermarket with bonus Viking ruins in Dublin: https://www.atlasobscura.com/places/aungier-street-lidl-arch...

In the same vein, Bloomberg office in London UK has an Ancient Rome museum and the access is free: https://www.bloomberg.org/arts/advancing-the-arts-around-wor... (disclosure: I work for Bloomberg).

Thessaloniki had the same issue, and now there's a stop where you have walkways above the ruins.

Some photos of the "before" here:

https://www.thessalonikiguide.gr/metro-thessalonikis-mia-arx...