Indonesian Navy submarine KRI Nanggala (402) sinks in the Bali Sea during a military drill, killing all 53 on board.

The Indonesian Navy is the naval branch of the Indonesian National Armed Forces. It was founded on 10 September 1945 and has a role to patrol Indonesia's lengthy coastline, to enforce and patrol the territorial waters and Exclusive Economic Zone (EEZ) of Indonesia, to protect Indonesia's maritime strategic interests, to protect the islands surrounding Indonesia, and to defend against seaborne threats.

KRI Nanggala (402), also known as Nanggala II, was one of two Cakra-class Type 209/1300 diesel-electric attack submarines of the Indonesian Navy. It sank following an implosion in April 2021.

The Bali Sea is the body of water north of the island of Bali and south of Kangean Island in Indonesia. The sea forms the south-west part of the Flores Sea, and the Madura Strait opens into it from the west.

April 21 is the 111th day of the year in the Gregorian calendar; 254 days remain until the end of the year.