[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.