U-166 was one of two U-Boat wrecks in the Gulf of Mexico. The boat was sunk early on in World War II, sinking only four ships before being destroyed by a U.S. Naval vessel next to its final target, the Robert E. Lee, in 1942. German submarine U-166 was a Type IXC U-boat of Nazi Germany's Kriegsmarine during World War II

U-166 departed from Lorient on 17 June 1942, sailed across the Atlantic and into the Gulf of Mexico. Robert E. Lee was under escort from the United States Navy patrol craft PC-566 approximately 45 miles (72 km) south of the Mississippi River Delta when she was torpedoed by U-166 on 30 July 1942. PC-566 immediately attacked and claimed to have sunk the U-boat with depth charges.

On 1 August 1942, a United States Coast Guard J4F-1 Widgeon amphibious aircraft spotted a U-boat approximately 100 miles (160 km) off the coast of Houma, Louisiana. The aircraft attacked and it appeared that the U-boat was hit in the attack. U-166 was reported missing in action on 30 July 1942, which coincided with the American aircraft's attack on "a U-Boat", leading to the aircraft being credited with the sinking of U-166, with the loss of all 52 crew members.

The site where U-166 lies has been designated a war grave. Discovered and identified in 2001 by Dan Warren and Rob Church of C&C Technologies, the U-166 rests in 5,000 ft of water some 50 mi southeast of New Orleans. Although the 2001 expedition positively identified the vessel as the U-166, time and equipment limitations prevented proper documentation of the site.