After World War II ended, the fighting forces had to figure out what to do with surplus military goods. Ships were scrapped or sunk, vehicles were sold at auction, and surpluses were sent to warehouses or auctioned out to resellers.
The Navy had a large supply of sodium that it had to get rid of. During the war, sodium was used to assist in the liquid-cooling process of large engines, in the manufacture of rocket fuel, and to purify molten metals like the steel used for Navy ships.
While most people think of sodium as something to worry about in their daily diets, it is actually a dangerous and explosive element when it's not bonded with something else. That being the case, the Navy decided to get rid of it by dumping it into lakes.
The chemical reaction between the sodium and the water releases a lot of hydrogen gas and heat. (You may remember hydrogen gas from the Hindenburg disaster.) The gas is then detonated by the heat of the reaction, causing a massive explosion.
See the intense results in the video below: