El Cajon Underwater Sinkhole.
Karst geology features are prominent in and around El Cajon Bay. The bay itself may have been formed by the collapse of the bedrock foundation. Earth cracks are common around the bedrock north and west rim of the bay. A large sinkhole in the bay is the outlet of an underground stream. Water discharge from the outlet empties into Lake Huron and, due to the constant flow and warmer temperature of water, the sinkhole never freezes over. When water levels in Lake Huron are high, the El Cajon sinkhole is submerged, but still visible on aerial photos. During low water levels like recent years, one can walk up to the edge of the sinkhole. The sinkhole discharge water has a different chemical composition than surface water of the bay and has a noticeable sulfur-like aroma. The photo below shows El Cajon Bay and the prominent sinkhole. Other sinkholes are located in Misery Bay and north in the Rock Port area.