It's dark in there. Bring light. If you have only temporary light sources such as torches, candles or flashlights, bring several. You do *not* want to be there in the dark.
There's no food there. Bring plenty. Really, lots.
Go in groups. If somebody gets hurt or exhausted, somebody else might be able to find help. It's good to have extra bodies too in case you find something interesting: might be too heavy for a single person to carry it all.