What is down there
Caves open to the surface through a few mouths and run in levels. The upper caves are chambers joined by winding tunnels. A shaft and a stair of dug steps lead down to the lower caves, with great halls and pools, and below them a deep level of wide, high chambers. Passages dip, narrow and swell as they go.
Some passages are meant for people and not for the cave's dwellers: squeezes, ledges, chasms and pits. See Adventure tunnels.
Light
Caves are pitch dark. Hold a Torches or mount torches on the walls as you go. If it is too dark for you, Settings, Comfort, View can make caves dim or visible instead.
CarefulA lit torch can draw Dripwings from far off.
Ore
Caves cut through ore pockets, so they are the quickest way to copper, iron and gold. The deeper you go, the better it gets. See Finding ore.
Who lives there
The deeper you go, the more creatures and the stronger they get: Hollows, Spirecrawlers and Pitcoils near the top, the cave pouncer and Dripwings further down, and the Vaultmaw, Riftstalker, Earwight and Veilgrip in the deep. Bats roost on the ceilings. A lair you clear is taken again while you are away.
Fallen satchels in caves hold things worth the trip.