At 06.06 hours on 18 May 1942 the unescorted and unarmed Mercury Sun (Master Willard Davis Jr.) was hit by two torpedoes from U-125 about 125 miles south of Cape Corrientes, Cuba, while steaming a zigzag course at 8.5 knots. The U-boat had chased the tanker for more than 13 hours before getting into a favourable attack position. The torpedoes struck on the port side at the #4 and #5 tanks and broke the back of the ship. The carbon dioxide smothering system on the tanker successfully kept the cargo from igniting after the first torpedo hit, but when the second hit the ship burst into flames. 29 of the nine officers and 26 crewmen managed to abandon ship in two lifeboats. At 06.35 hours, a coup de grâce struck the vessel on the starboard side at #8 tank, but the ship remained afloat and sank about three hours after the first attack, sagging in the middle. The master, chief mate, second mate and three crew members were lost.

The lifeboats stayed near the burning ship until daybreak and then sailed towards the coast. 28 survivors were picked up nearly 40 hours after the attack by the American steam merchant Howard and landed on 19 May at Mobile, Alabama. One seriously injured crewman was transferred to a US Coast Guard boat at the Tampa Sea Buoy.