Fluctuating sea levels compressed numerous layers of calcium carbonate, sand, and shells. The resulting permeable limestone formations that developed between 25 million and 70 million years ago created the Floridan Aquifer, which serves as the main source of fresh water for the northern portion of Florida. However, this aquifer lies beneath thousands of feet of impermeable sedimentary rock from Lake Okeechobee to the southern tip of the peninsula.[12]