In an expansive white-tiled room in Livermore, California sits Sierra, the world’s second most powerful supercomputer. Sierra looks like an unassuming server farm, but is actually a massive connected hive of 190,000 processing cores. It was completed earlier this year, and has been on a shakedown cruise since then: researchers at the Lawrence Livermore National Laboratory ran astrophysics, climate, and precision medicine simulations on Sierra while ferreting out bad components and other technical hiccups.

But early next year, Sierra’s real work will begin. The system will be “air gapped,” meaning that it will be disconnected from any external network to prevent unauthorized access. Once that happens, it can begin the calculations it was purpose-built to carry out: simulations of nuclear weapons launches and detonations.

The exact nature of the simulations, not surprisingly, is classified. But for one day, Livermore Lab allowed The Verge and other press access to the secure facility. We got to see Sierra and speak with the engineers and physicists tasked with detonating virtual nukes in the name of national security. Check out the video above to find out what we learned, and why the United States needs a supercomputer to support its enormous nuclear stockpile.