An Unidentified Flying Object, or UFO, was a term in use on Earth during the 20th and 21st centuries to explain the presence of an object in Earth's atmosphere or in orbit that could not be explained by the technology in use at the time. Many incidents of UFO sightings were later revealed to be vessels from the 23rd or 24th centuries that had traveled back in time.
In 1947, a vessel owned by Quark crashed in the New Mexico desert near the city of Roswell after being sent back in time from 2372. Considered a UFO by the United States Army, the vessel was impounded by the American forces, and the crew - consisting of Quark, Rom, and Nog - were held by the Americans at Roswell Army Air Field. Odo had also accompanied the Ferengi, but as he was disguised was able to evade capture. With the assistance of Faith Garland and Jeffrey Carlson the visitors from the 24th century were able to escape. The United States later covered up the entire incident by claiming a weather balloon had crashed. (DS9 episode: "Little Green Men")
Gary Seven has mitigated many UFO incidents to prevent a planetwide panic because of aliens visiting the planet. To accomplish the missions, he often had to disguise himself, other agents AS high ranking officers, and others who had access to highly classified matters, so as to mitigate the many UFO incidents that have been going on. (TOS novels: The Rise and Fall of Khan Noonien Singh, Volume 1, The Rise and Fall of Khan Noonien Singh, Volume 2)
The USS Enterprise had traveled back in time to 1969 after escaping the gravity of a black star. They had finally regained control of the vessel while in Earth's atmosphere. Meanwhile, United States Air Force officials on the ground were tracking the Enterprise, and sent Captain John Christopher up to investigate this large UFO in Earth's upper atmosphere. (TOS episode: "Tomorrow is Yesterday") This incident was also mitigated by Gary Seven, since all USAF bases have several sensor recording and radar recording facilities. (TOS - Star Trek: Assignment: Earth comic: "Yesterday, Today and Tomorrow")
In 1986, a Klingon Bird-of-Prey under command of the senior staff of the late USS Enterprise decloaked in the Bering Sea. This ship, named the HMS Bounty, was able to prevent the humpback whales George and Gracie from being killed by a whaler operating in the area. A tracking station in Nome, Alaska tracked the Bounty as it left Earth's atmosphere. Gary Seven had the radar and sensor logs seized and destroyed to preserve the timeline. The whaling captain and crew's UFO sighting was discredited, since alcohol (beer, whiskey, vodka) was found, and some of the crew were deemed mentally ill in accordance to the Robertson Panel. (TOS movie: Star Trek IV: The Voyage Home)
When USS Voyager traveled back to 1996, the ship briefly entered the atmosphere in an attempt to retrieve a 29th century timeship before Henry Starling could use the ship to travel to the 29th century. Voyager passed over Los Angeles, and was captured on video by a number of people, which was subsequently broadcast on news stations. The United States government downplayed this UFO sighting, however the United States Armed Forces were placed on alert. (VOY episode: "Future's End") After Voyager left, Gary Seven also mitigated this UFO incident, to preserve the timeline. He also found and completely destroyed the crashed shuttle that Voyager had sent, after its remains were sent by the US military to Area 51. To "explain" what people had videotaped, Gary Seven issued statements to CNN, and other news outlets that the object photographed and videotaped was that the USAF was testing a new plane, a possible replacement for the B-2 Spirit, and that the flight crew were disciplined for flying the plane over a populated area. The "test plane" was at Edwards Air Force Base (which is near Los Angeles) then was "taken" to "a undisclosed military base". (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 2)
Later that year, Khan Singh's ship was launched from Area 51, and as it went into outer space, it flew over Nevada, then Arizona, which was promptly reported as one of the UFOs seen during the Phoenix UFO Incident. Alien ships were also seen, including one that was the size of a large city hovering over Phoenix, Arizona. Gary Seven mitigated this famous "UFO Incident" with great difficulty. He even had someone dressed as a alien, placed in handcuffs, escorted by Arizona DPS police officers put on TV as a joke. The other UFOs that were videotaped were top secret craft and holographic effects. Those flying the top secret aircraft were severely disciplined for flying these over populated areas, according to statements given by Gary Seven to news outlets of the era, such as CNN, FOX TV News, and FOX News, to preserve both the timeline and to prevent panic. (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 2)