Avery Island is an extruded bubble of a salt dome formed from the salt of an ancient Jurassic sea that left a vast bed of salt far below the marshy Louisiana coast. Over the millennia, the Mississippi River laid down layer after layer of sediment over this salt bed, and the weight of that sediment caused the more buoyant salt to rise through unconsolidated places in the overlying sediment, much like toothpaste being squeezed from a tube, creating domes of salt below the Gulf Coast's soft subsurface. The Avery Island salt dome has broken through the earth's surface, rising over 150 feet above sea level, and is estimated to be over 26,000 feet deep, nearly the size of Mount Everest. During frequent rains along the Louisiana coast, water runs off Avery Island's hills with enough volume and speed to erode the relatively thin layer of topsoil, and Sunken Gardens was McIlhenny's attempt to slow the rush of water during heavy rains while creating a quiet garden in the process.