The phrase "to enter the cave" means going inside a dark and enclosed space. It can also be used metaphorically to describe facing challenges or confronting the unknown. Full definition
You can pick up to three characters to enter the cave with you, and each of them has their own abilities that will help you through the levels. (plusxp.com)
In 2003, another animal entered the cave — with a ladder. (discovermagazine.com)
[10] Parties continue to be a problem when individuals enter the caves and litter the ground with garbage, broken glass, and human feces. (en.wikipedia.org)