Hormiguero is a rather large (though not daunting) Rio Bec-style site which especially features the famous Monster Mouth entrances. These are fantastically decorated friezes depicting Itzamna, the terrible Mayan god, Monster of the Earth...A door represents its mouth, and the upper friezes represent its eyes and eyebrows; the bottom sill is decorated with curved stones, representing its teeth, the side friezes represents its ears, etc. Fantastic stuff. The Mayans believed that one who entered by the door (the mouth, remember?), actually entered Itzamnal's body and therefore, the entrance to the underworld.
Unfortunately, the road to Hormiguero is REALLY terrible with hundreds of potholes and broken asphalt, making the trip there pretty harrowing. Yet, for Mayan ruins fans, it's hard to pass up!