The rock on which Orvieto sits is volcanic and soft, so it's easy to tunnel into. Combined with the fact that space was limited on the rock, residents created space underground. They used the space to press olives to make olive oil, to store water, to make rope, for dovecotes, and for other things. Here are some photos from a tour I took.

Black indicates the above-ground structures. Red indicates the underground spaces, but the map is not complete:

This is an Etruscan well, of which 40 have been found in Orvieto. They were the first underground spaces to be created. As you can see, this one goes a long way down, and it hasn't been cleaned it out completely, so it goes down even farther. You can barely see the foot holds on the bottom side, which were used by the diggers to get in and out. It is thought that bellows were used to get fresh air to the digger when the well was being dug.

This is a room was used to press olives to make olive oil. The press is off to the left. A draft animal was used to turn the milling stone. Before being an olive mill, it's possible the Etruscans used this room as a chapel.

These are dovecotes. These rooms were on the outer wall, so they could have an opening to the outside that was always open, allowing the pigeons to come and go. The pigeons were a food source that required almost no work, which made them pretty popular.

This photo shows the tunnel that provided the owner access to his dovecote. It's blocked off now, but at one time it went to the surface, perhaps inside the owner's house.