This is the excavation at Ercolano (or Herculaneum, depending on your language preference). Pompeii has all been excavated to the same level, but here you can see how much higher the current city is than the ancient city.

It's also why so little of Ercolano has been excavated -- they'd have to tear down half the city. So instead, they've been building tunnels from the excavated part into the rest of the city underneath the houses you can see in this picture.