Keys, has a surface area of about 572 km2. The larger lagoon, including Card Sound and Barnes Sound, has a surface area of about 703 km2. It has an average depth of 1.8 m and, except where channels have been dredged, a maximum depth of 4 m.

The eastern rim of the lagoon is based on an ancient coral reef which existed along the southeastern edge of the Florida Platform about 100,000 years ago. The ensuing Wisconsin glaciation