The explosion lit up the sky. It cracked windows. It jolted San Francisco residents awake from their sleep. It killed 320 men instantly and injured nearly 400 others. This was the worst home front disaster of World War II. And it took place on the night of July 17, 1944, at the Port Chicago Naval Magazine in Contra Costa County, California.

On that night, crews at the Port Chicago Naval Magazine, about 30 miles northeast of San Francisco, were loading two naval vessels for Pacific theater troops with active ammunition and bombs. The explosives ignited, killing hundreds, obliterating the ships and the pier, and creating a blast that registered a 3.4 on the Richter scale.

Of the 320 who were killed, 202 were young African-American men. These enlisted sailors were working for a segregated military. They lived in segregated barracks, were denied meals until after the white military personnel had left the mess hall, and, despite their training, were assigned to positions in the kitchens or as stevedores (dockworkers who loaded and unloaded ships).

These men at Port Chicago received little to no formal training in the proper handling of explosives. In fact, they were told that the munitions they were handling were all inactive. The explosion of July 17, 1944, brought all of this to light.

Four days after the explosion, a Naval Board of Inquiry was convened. It lasted 39 days. Interviews were conducted with witnesses, and ordnance experts and inspectors were consulted. There were several possible scenarios explored, including sabotage, faulty fueling procedures, defects in the munitions, and improper handling by the loaders.

The investigators concluded that the cause of the explosion could not be determined, and each victim’s family received $3,000 in compensation.

A memorial service was held on July 31, 1944, at Port Chicago.

On August 8, 1944, 328 of the African-American sailors were asked to load naval mines and other munitions on the USS Sangay, docked at the Mare Island Navy Yard in nearby Vallejo, California. No additional training was given to prevent another disaster like the one at Port Chicago, and no changes in operations were made.

The sailors refused.

Facing threats of imprisonment or death sentences for mutinous conduct, many later agreed to the work. Those who continued to refuse were taken to a military prison.

Ultimately, 50 men—known as the “Port Chicago 50”—were taken to trial in what was the largest naval trial of its kind in U.S. history. All 50 were convicted of mutiny.

The explosion at Port Chicago and the ensuing mutiny had a huge and lasting impact. The Navy made significant changes to how munitions are designed and handled. And in the wake of Port Chicago, the Navy began integrating its regiments in June 1945. In 1948, the entire U.S. military was desegregated.

Today, the Port Chicago Naval Magazine National Memorial stands at the site of the explosion. Established in 1992 and dedicated in 1994, it honors those who lost their lives and reminds visitors of the explosion and its political and social effects. It also stands as a reminder of the nation’s continued struggles for equality and social justice.

This year marks 73 years since the explosion. An event marking the anniversary is being held at the memorial on July 15, 2017.