Try spelunking at the impressive limestone caverns of Ak'tun Kan, which translates from Q'eqchi' Maya as 'Cave of the Serpent'. The cave-keeper provides the authorized interpretation of the weirdly shaped stalagmite and stalactite formations, including the Frozen Falls, the Whale's Tail, and the Gate of Heaven, the last within a great hall where bats flutter in the crevices.
If you haven't got a flashlight, you can rent one. Explorations take 30 to 45 minutes. It's 2km south of Flores; take a tuk-tuk there (Q10).
