An American hydrogen bomb is still missing over 50 years after a jet crash off Greenland, a Danish territory where the United States has maintained military presence since World War II.

Tuesday, January 21, 2020, marked the anniversary of a US military plane crash that went on to become one of the most serious tragedies in the history of nuclear weapons.

On that special date, a US strategic B-52 bomber crashed seven hours after taking off from Thule base in Greenland.

The incident caused a huge nuclear contamination in the region as the bomber was armed with four hydrogen bombs.

However, five decades have passed and US authorities have yet to find a fourth bomb carried by the B-52 bomber. Submarines sent to the site of the crash in 1968 recovered debris of the other bombs, including the secondary of a bomb that had remained intact.

A Danish newspaper reported in 2006 that the bomb might have remained unexploded on the seabed off Thule, an air and radar base in the Arctic with a huge significance to the US military.

“Detective work by a group of former Thule workers indicates that an unexploded nuclear bomb probably still lies on the seabed off Thule,” said the report by Jyllands-Posten, a mass-circulation daily.

The case of the bomb and other nuclear-related issues have caused sporadic tensions between the US and NATO ally Denmark. Information that emerged in recent decades suggested that the Nordic country had been kept in dark about secret US nuclear activities in Greenland, including about Camp Century, a nukes silo built in 1959 deep under Greenland’s ice sheet.

That comes as people and the devolved government of Greenland, which operates under Denmark’s rule, are generally unhappy with the inaction on cleanup of nuclear waste left over in their island by the Americans.

The Greenland government said in a statement in late 2016 that Washington and Copenhagen had yet to come up with proper explanations about the environmental and health implications of waste in more than 30 abandoned American military installations in the island.