A vast marshland could only have been formed due to the underlying rock formations in southern Florida.[15] The floor of the Everglades formed between 25 million and 2 million years ago when the Florida peninsula was a shallow sea floor. The peninsula has been covered by sea water at least seven times since the earliest bedrock formation.[15] The rock that makes up the Everglades floor was created as layers of calcium carbonate were compressed by ocean water, making limestone. Fossilized bryozoans and tiny shells, or ooids, make the limestone porous. Water is stored in the rock, sometimes from one year to the next.[16] The length of time that a region in the Everglades remains flooded, called a hydroperiod,[4] determines what particular soils and vegetation are present.