If you want a base down there: Tame as many bulbdogs/shinehorns/glowtails as possible. Keep them scattered all over your base. Do not let a single area be unilluminated or you'll have a Nameless infestation... Do not be down there without a Crab and a pack of ravagers.
If you're just exploring: At least 10 pairs of climbing hooks. Make sure you have high fortitude, a hazmat suit and at least 400 Aquatic and Agavic mushrooms. a shotgun or assault rifle will do you well.