Although tensions between the United States and Russia have increased in recent years due in large part to Russian disinformation and espionage operations the world over, these two nations still maintain a number of security-centric treaties meant to diffuse the possibility of open war. Some of these treaties have been very publicly tossed aside in recent months, but others, like the 1992 Open Skies Treaty, remain an active (and often contested) part of America’s tumultuous relationship with Putin’s Russia.

The Open Skies Treaty is an agreement established between 34 signatory nations that allows for unarmed and observed military surveillance flights over one another’s nations. In short, it grants these nations stipulated permissions to fly spy planes through one another’s airspace to keep track of various elements of any nation’s military apparatus. Russia and the United States, both signatory members of the treaty, use this agreement to fly their reconnaissance aircraft over areas of strategic interest, but those flights must come with prior approval from the host nation and usually include local observers flying on board the surveillance flights. Strict regulations are meant to ensure only certain types of intelligence are gathered, and the local observers ensure these flights abide by the terms that have been agreed to ahead of time.

In 2017, for instance, a Russian Tu-154 was spotted by nervous spectators flying as low as 3,700 feet over Washington D.C., gathering intelligence data for the Kremlin. Soon thereafter, however, Russia attempted to curtail America’s planned Open Skies flights, resulting in months of diplomatic squabbling.

Last week, for the first time since 2017, a Russian Tu-154 was once again spotted operating over the American mainland thanks to the Open Skies Treaty, but rather than flying over the nation’s capital and Camp David as they have in the past, these flights included areas of unusual interest, such as a small airstrip in the Groom Lake Dry Bed nestled in the Air Force’s Nevada Test and Training Range. That airstrip and its associated facilities are better known colloquially as Area 51.

In recent decades, the name Area 51 has become synonymous with tales of captured alien spacecraft and even experimentation on aliens themselves. Its common use throughout pop culture has actually prompted some in the UFO-believer community to suggest that making Area 51 into a joke of sorts has been encouraged by the government itself, as it makes stories about unusual aircraft spotted in the skies above Groom Lake easier to dismiss.

The truth is, this small airstrip has actually been an integral part of testing some of the most advanced air platforms this planet has ever seen. From the U-2 spy plane to the F-117 Nighthawk, nearly all of America’s most capable (and secretive) aircraft have spent time being tested at Area 51. As a result, UFO sightings in the area are only logical, especially in previous decades when unusual-looking stealth aircraft had never been seen anywhere before.

It seems likely that it was an awareness of America’s airframe testing at Groom Lake, rather than the possibility of encountering alien spacecraft, that prompted the Russian interest in this flight. As reported by Tyler Rogoway in The War Zone, the Russian Tu-154 likely took off from Travis Air Force Base in northern California before touring a number of U.S. military installations in the Southwest. Its flight also included passes over Naval Air Station Lemoore and Naval Air Weapons Station China Lake among others before swinging back north and heading into the highly scrutinized airspace of the Nevada Test and Training Range.

The flight didn’t directly pass over the Area 51 airstrip and hangars, but it did fly close enough to gather optical intelligence on the entire area from an altitude of around 15,000 feet.

Last year, satellite imagery confirmed that a large new hangar has been constructed on the Area 51 airstrip, suggesting that the base is still in use and either has or will likely soon see a new airframe being tested that may be too large for the existing hangars to accommodate. The size and location of this new construction has led some to postulate that early test flights of the forthcoming B-21 Raider may be conducted out of Groom Lake.