Cobá includes the Great Pyramid which is the highest Mayan structure in the Yucatán.  

It is north of Tulum in Quintana Roo.  This site has not been restored, and is the largest Classical Mayan city.  At one time, there may have been a population of 40,000 Mayans residing there.  There were mysterious roads built out of Cobá which now seem to lead nowhere in particular.  
 

Cobá is located in the middle of the jungle, to get to these beautiful ruins one needs to walk about two miles.  The pyramids in Cobá were built in layers, every 52 years another layer was built on top of the existing layers.