The US military is in a state of "heightened alert" over unidentified objects flying in its airspace, with no less than four of them having been shot down in the past eight days.
Only the first one of those, a white balloon determined to be a surveillance object sent from China, has been identified as of yet, while the origin of the rest remains a mystery as of yet.
But it is not the first time that the Pentagon is faced with unexplained floating objects in its airspace and officials struggle to come up with an explanation.
From the summer of 2014 to March 2015, strange objects, one of which was described as a spinning top moving against the wind, were seen almost daily in the sky over the East Coast.
According to reports from Navy pilots, these objects had no visible engines or infrared exhaust plumes, yet were capable of reaching 30,000ft and hypersonic speeds.
Former Navy Lt. Ryan Graves, a 10-year veteran pilot of the F/A-18 Super Hornet, stated that these objects were present in the air for extended periods of time, which was surprising given the energy required to keep an aircraft in flight.
In late 2014, one of the objects nearly collided with a Super Hornet, and an official report was filed, the New York Times previously reported.
Some of the encounters were recorded on video, including one from early 2015 that showed an object soaring over the ocean waves, leaving the pilots in disbelief.
One of them, stunned, was heard exclaiming: "Wow, what is that, man? Look at it fly!"
Despite speculation about the objects' origin, no one in the Defense Department has claimed that they were extraterrestrial, and experts believe that there are usually terrestrial explanations for such events.
Navy pilots did not speculate about where the objects came from, however, the Navy took notice at the time and issued classified guidance for reporting these unexplained aerial phenomena.
According to Joseph Gradisher, a spokesperson for the Navy, the recent update to the guidelines for reporting suspected intrusions into the country's airspace was a revision of the instructions issued to the fleet back in 2015, following the incidents involving the Roosevelt.
Gradisher stated that there were various reports of these sightings and that while some could possibly be attributed to commercial drones, in other instances, there is not enough data available to identify the source.
The updated guidance was provided to the fleet in an effort to standardise the reporting procedures.
The sightings were reported to the Pentagon's Advanced Aerospace Threat Identification Program, a lesser-known program that analysed radar data, video footage, and accounts from senior officers on the Roosevelt.
The program's former head, Luis Elizondo, who resigned in 2017, referred to the sightings as "a striking series of incidents."
The Advanced Aerospace Threat Identification Program, initiated in 2007 at the request of former Senate Majority Leader Harry Reid, a Democrat from Nevada, was officially discontinued in 2012 due to a lack of funding, as per the Pentagon.
However, the Navy acknowledged in 2019 that it continued to investigate military reports of UFOs.
One of the case studies that it looked into, was a video showing a whitish oval object described as a "giant Tic Tac".
Two Navy fighter jets spotted the object, which they said was about the size of a commercial plane, off the coast of San Diego in 2004.
But a senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics said the possibility of an extraterrestrial cause is unlikely.
He told the New York Times: "[It] is so unlikely that it competes with many other low-probability but more mundane explanations.
"There are so many other possibilities — bugs in the code for the imaging and display systems, atmospheric effects and reflections, neurological overload from multiple inputs during high-speed flight."
