Filed under "incredibly cool, but only after I manage to drag my ass to the gym more regularly", is Sun Doong cave in Vietnam. At over 5 miles long and 500 feet high in places, the cave is actually the worlds largest. It requires hours of hiking to reach the cave, and with all that space to explore, the tours offered are a 7 day ordeal, err, adventure.
It looks like it's worth the trek! There's so much more to see than the usual stalagmites and stalactites. Holes in the ceiling of the cave let in light and have allowed an incredible ecosystem to develop: