Found this little gem on the maps we bought from Canmaps.com. We drove a little way out of town, past a guarded gate (don't worry, the guard just lets you by), to this little tiny parking lot.
Pay about five pesos and you get to put on a helmet with a light on it. Our tour guide did not speak a word of English. When we announced "no comprende," he promptly switched to Mayan!! It was a wonderful tour, even though we couldn't understand our young guide.
After the tour, you stop in a wonderful naturally lit part of the cave for refreshments. We were the only ones here and I don't think many of the tourists even know it is there.