Second-deepest "big blue hole" found in the ocean

A new giant underwater cave has been found off the coast of Mexico.

Scientists from Colegio de la Frontera Sur, in Mexico, have discovered the second-largest "blue hole" in the ocean. Blue holes are a bit like sinkholes on land. They are made in limestone rocks, which easily dissolve in freshwater. When water enters cracks in the limestone, the gaps get bigger until they eventually form an open passage. Over thousands of years, the water slowly wears away the rock, leaving vast chasms and caves. At the end of the last ice age (around 11,500 years ago), ice covering the land melted, making sea levels rise. Seawater then filled the caves, leaving deep blue holes in the middle of the ocean.