The Chernobyl disaster: Reactor number four at the Chernobyl nuclear power plant, in the Soviet Socialist Republic of Ukraine, melts down during a safety test gone awry.

       [added 12/9/2022]

Subsequent Events:

4/21/1988                   7/7/1989                    12/26/1991

References:

Revolutions of 1989 – Wikipedia
en.wikipedia.org/wiki/Revolutions_of_1989

Chernobyl disaster – Wikipedia
en.wikipedia.org/wiki/Chernobyl_disaster

Current U.s. National Debt:

$36,167,124,467,492

Source