Looks like it could be an ancient burial mound, designed like a crypt. Hope the party remembered to bring a cleric!
I like it. faoladh's suggestion of ancient burial site is great.
I like the idea the party keep popping out on different sides, and the confusion in the mapping. You might need a set of landmarks for the horizon through 360 degrees - or not...
Why am I suddenly hungry for... braaaaaaains?
Very nice! Lots of good ideas too - the hill, canyon, burial mound (even a brain - "Fantastic Voyage," anyone?), all of them would work quite well.
In all seriousness this time, these look like the worm tunnels a we've been working on at my blog http://www.digitalorc.blogspot.com/2012/03/map-worm-tunnels.html
I'm imagining it is a meteorite that crashed into the surface of a swampy area. The "caverns" and "labyrinths," if you will, are cracks in the surface of the meteorite. And I'm imagining that in the middle of the meteorite sleeps a curled up "godlet" that some cult is trying to wake up. Why am I thinking about this? Well, I was just reading about the Tunguska event in 1908, when part of a comet slammed into northern Russia.
I immediately thought of an archipelago. The type where pirates can hide and no one can go after them.