Adventurers traveling to a remote jungles to enter a gigantic cave, inhabited by things like poisonous centipedes isn't just the stuff of table top fantasy. The February issue of National Geographic has an profusely illustrated article about an epedition to Son Doong Cave in Vietnam. The cave's at least 2.8 miles long and is some places as tall as 460 feet. Check out all the pictures and an interactive map here.
Not enough for a jaded delver like yourself? Well, marvel at the the crystal formations in Mexico's appropriately named Cave of Crystals. If that's not an adventurous environment (minus, you know, the extreme heat of 136 F and 90-100% humidity) I don't know what is:
Note the size of the people in relation to the crystals!