Underground tunnels

One of the current missions at the Nevada National Security Site is to conduct what are known as "subcritical experiments." This is because the U.S. still maintains a large stockpile of nuclear weapons, and under the U.S. Department of Energy and National Nuclear Security Administration's Stockpile Stewardship Programs, the government must "maintain the safety and reliability" of that stockpile.

At a site called U1a, scientists have created a large network of underground tunnels where they conduct these experiments. Each test is performed in a side tunnel, and generally, a small amount of plutonium will be bombarded with some kind of material in order to smash it and release energy. This is what is known as the subcritical experiment. After finishing each experiment, that side tunnel is closed off and filled with concrete.