Subsidence craters left by underground tests in Nevada. (Photo: Dept. of Energy)
North Korea has conducted an underground test of a nuclear weapon, its fifth in a decade. According to South Korea, which measured the force of the resulting earthquake, the bomb being tested was the most powerful yet, the equivalent of 10 kilotons of TNT, the New York Times reports.
Since the 1963 Test Ban Treaty, the world's major nuclear powers have tested their weapons underground. The treaty barred nuclear testing in the atmosphere, in space, or underwater. In the decades that followed, the U.S. and the Soviet Union conducted hundreds of underground nuclear tests; all in all, from 1945 to 1998, the U.S. performed 215 tests above ground and 815 underground.
The aim of the test ban treaty was to limit fallout in the atmosphere and exposure to radioactive materials. When a weapon buried deep enough, its explosion can be contained in the ground; how deep depends on how big the bomb is. If the bomb is not buried deep enough into the ground, it will not necessarily produce a classic mushroom cloud, but it will explode a giant cloud of dust and dirt into the sky, as seen above.
The largest underground test the U.S. ever conducted, Cannikin, used a 5 megaton bomb, which was buried more than 6,000 feet below the surface of the earth. Even then, the force of the explosion lifted the ground twenty feet. Here's what that looked like:
Or here's a shorter version with more dramatic music:
Underground tests still risk contaminating the land or water surrounding the test site. Cannikin took place on Amchitka Island, in Alaska's Aleutian islands, and the Department of Energy Office of Legacy Management still monitors environmental conditions there. After the last monitoring visit, in 2011, the DOE announced that seafood harvested around the island was safe to eat; the next monitoring visit is scheduled for this year.