About a kilometer before Semuc Champey, just before the large bridge crossing the river, you'll see a turnoff to the right for these caves, which many find to be much more interesting than Grutas de LanquÃn. Bring a flashlight for the two-hour tour or you'll be stumbling around by candlelight.
A half hour of river tubing costs an extra Q10. There have been reports of serious overcrowding on these cave tours. If you can, arrive for the 9am tour, before the tour groups start showing up. And, as always, evaluate the conditions before setting out.