(NewsNation) — A Chinese balloon captured national attention as it flew over the U.S. before being shot down over the Atlantic ocean. But how many surveillance balloons are out there?

U.S. officials said Saturday that similar Chinese balloons transited the continental United States briefly at least three times during the Trump administration and once that they know about earlier in the Biden administration. But none of those incidents lasted the length of time or garnered as much attention as this latest, which drifted across the U.S. for several days before it was shot down.

Balloons have been in use for decades for surveillance and other purposes, though they came into more significant use during WWII.

During World War II, Japan launched thousands of hydrogen balloons carrying bombs, and hundreds ended up in the U.S. and Canada. Most were ineffective, but one was lethal. In May 1945, six civilians died when they found one of the balloons on the ground in Oregon, and it exploded.

In the aftermath of the war, America’s own balloon effort ignited the alien stories and lore linked to Roswell, New Mexico.

According to military research documents and studies, the U.S. began using giant trains of balloons and sensors that were strung together and stretched more than 600 feet as part of an early effort to detect Soviet missile launches during the post-World War II era. They called it Project Mogul.

One of the balloon trains crash-landed at the Roswell Army Airfield in 1947, and Air Force personnel unaware of the program found debris. The unusual experimental equipment made it difficult to identify, leaving the airmen with unanswered questions that over time — aided by UFO enthusiasts — took on a life of their own. The simple answer, according to the military reports, was just over the Sacramento Mountains at the Project Mogul launch site in Alamogordo.

As time passed, more sophisticated intelligence-gathering technology took hold, including the use of satellites and drones.

So why would a country like China use a balloon when other intelligence-gathering tools exist? One reason might be to test the political response.

Balloons also offer other advantages, including being cheaper than satellites or drones. Because of their lower altitude, they can get higher-quality images and are also more maneuverable than other technology.

U.S. officials said the most recent balloon, which the Chinese government maintains was being used for meteorological research, would not have been able to get any intelligence China wasn’t already able to gather through other means. However, the government still took steps to reduce the amount of information the balloon could collect on its flight.

Recent reports on unidentified aerial phenomena show the Pentagon had an increase in reports and since 2021, 163 reports of UAPs were later identified as balloons.

Fox News reporter Lucas Tomlinson reported that a similar balloon crashed off the coast of Hawaii four months ago, per U.S. officials.

Pentagon officials also confirmed a second Chinese balloon was believed to be looming over Latin America.

Of course, China isn’t the only country to use balloons. The U.S. government has also continued to use and test balloons for various purposes, including intelligence gathering.

Since governments aren’t eager to share the ways they gather intelligence from other countries, it’s hard to say exactly how many balloons are out there at any given time.

But it might be more than you think.

The Associated Press contributed to this report.