Operating under the Liberian flag, she ran aground on 16 March 1978 on Portsall Rocks, 2 km (1.2 mi) from the coast of Brittany, France.

[1][2] On 16 March 1978 in a southwesterly gale, the Amoco Cadiz passed Ushant at the western tip of Brittany, headed for Lyme Bay in the United Kingdom.

At 9:46 am when the supertanker was north of Ushant and 16 nautical miles (30 km; 18 mi) west of Portsall she turned to avoid another ship and her rudder jammed, full over to port.

By the time the tugboat Pacific successfully attached a hawser, it was 2:00 pm and the Amoco Cadiz had drifted 6 nautical miles (11 km; 6.9 mi) closer to the shore.

[4] The rock cut through the plating of her bottom and thrust into the network of piping and machinery of the pump room as well as rupturing the rear wall of number-four cargo tank.

[6] The Navy dropped twelve Mark 56 anti-submarine grenades, each containing 350 lb (158.757 kg) of high explosives set to go off 8 m (26 ft 2.96 in) under water, and she sank 15 minutes later.

[6][7][8][9] Amoco Cadiz contained 1,604,500 barrels (219,797 tons) of light crude oil from Ras Tanura, Saudi Arabia and Kharg Island, Iran.