You usually know what to expect from your vacation—the beach will be sandy, the accents will be alluring, and the minibar will be laughably inadequate.

But sometimes you just need to take a dangerous, exhilarating leap into the pitch-black unknown.

Literally.

Welcome to Aventuras Tierra Adentro, your chance to climb, zip-line and free-jump your way through some of the largest caves in the Caribbean, taking reservations now.

This is an underground trek incorporating all the spry athleticism and scuttling wildlife of Indiana Jones, minus the giant boulder. But first you've got to hike through the Caribbean jungle. You're seeking a series of five zip lines that'll send you careening across a ravine, over a winding stream called Rio Camuy... and straight into the mouth of a gaping 100-foot crater. At which point you may want to switch on that helmet lamp.

For the next few sunlight-free hours, you'll slog through lakes of mud, float down an underground river and scale 250-foot cliffs with just a carabiner and a few metal footholds to speed you along. Your guide has been around since the '80s, so you can trust him... even when he positions you at the edge of a cliff and tells you to jump blindly into the complete and utter darkness below.

Theoretically, you'll splash safely into an unseen underground lake.

Still, we'd try to stay on his good side.