The aim is to uncover whatever secrets are hidden there – but Area 51 is just a drop in the ocean when it comes to mysterious US bases.
Dozens of secretive facilities are dotted across the globe, and we've put together some of the strangest below.
Area 6 – Nevada, USA
This top secret airbase was spotted in the Nevada desert on Google Earth in 2016.
The site is roughly 12 miles northeast of Area 51 and is part of the Nevada National Security Site, where 1,000 nuclear tests were carried out between 1945 and 1995.
Six nuclear tests and four detonations have been completed at Area 6, according to the US Department of Energy.