When termination letters were sent to employees across the federal government last week, the Trump administration affected an agency charged with the readiness of America’s nuclear arsenal.

The move also shined a spotlight on the agency, the National Nuclear Security Administration, which few Americans likely think of often, if ever, but has a monumental responsibility.

Here’s a rundown of what the obscure agency does and the potential reasons that the Trump administration had to quickly adjust some of the firings:

What is the agency’s mission?

The National Nuclear Security Administration maintains, refurbishes and keeps safe the United States’ more than 3,000 nuclear warheads. It also supervises the production of new nuclear warheads. It has a $25 billion annual budget and more than 57,000 employees. Congress also put the agency in charge of thwarting nuclear proliferation, researching and developing nuclear propulsion systems for submarines and directing national laboratories that provide key scientific and engineering knowledge for the U.S. nuclear weapons system.

Those laboratories include the historical Los Alamos National Laboratory in New Mexico, where the top secret Manhattan Project took place during World War II.

To what department does it belong?

Confusingly, despite its long-held, pivotal role in the military’s defense strategy, the agency sits within the Department of Energy, rather than the Department of Defense.

In 2017, after accepting the offer to serve as energy secretary in the first Trump administration, Rick Perry was bewildered that the job entailed supervising the maintenance and production of the most fearsome weapons in the world.