> 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...

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...

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).

In Sofia we have roman ruins everywhere around metro stations https://archaeology-travel.com/bulgaria/serdika-ii-metro-sta...

It's quite interesting to see how much earth is typically above ancient ruins. Cities built upon cities for 1000's of years where a street or building was once at ground level and now is 2-4 stories beneath our modern world.

While visiting Vienna, there are ruins on display in Michaelerplatz (central Old City), so cool.

For reasons, I used to go to Rome quite frequently in the 2010s, and the construction of Metro C was already a meme. But now some of the stations are quite interesting indeed.

Okay, I'll say it: is it really worth encumbering the movements of millions of people for decades in order to make a few boring history exhibits? If you want to see some the bone comb that belonged to somebody's great^100-grandmother, there are dozens of museums that already have one on display.