Land bridge
Several kinds of change can cause a land bridge to rise above or sink below the water. Plate tectonics may cause the rise or lowering of land. The causes of general sea level changes are not fully understood, but often big polar ice caps lower sea level by locking up much of the water. When sea levels fall, shallow sections of continental shelf may get exposed. Sometimes the sea floor rises due to "post-glacial rebound" after an ice age.