During the last ice age (1.5 Million years ago), the appearance of Yucatan completely changed.

The ice caps at the poles grew, while the sea level sank about 100 meters.

At this time, the rain water carved wholes into the limestone ground, thus creating the space for the evolution of stalactites and stalagmites.

As the ice began to melt again (which led to a rising sea level), the caves were flooded with water again.

In some places, the underground water streams had washed away the soil, so the ground fell in. The famous Cenotes ("waterholes") evolved.

The water which fills the caves today, is partly intruding saltwater from the sea, but mainly fresh (rain)water.

Due to their different density and weight, saltwater and fresh water do not mix. The fresh water "floats" on the saltwater.