Mystery airships was a series of UFO sightings in the US between 1896 and 1897, making it the first massive UFO appearance in recorded history. A similar series of events took place (also in the US) between 1909 and 1912.
The UFOs sighted in 1896-97 closely resembled airships (hence the name) but vastly outperformed all airships built to date, including the Zeppelin LZ1 which was launched in 1900, starting the "Golden Age" of airships. Several individuals claimed to have build these highly advanced airships but no evidence was published after 1897.
In 1909-12, a similar wave ("flap") of UFOs was seen in the US, only this time it were fixed-wing aircraft instead of airships. Appearing in massive numbers in those years, they very similarly disappeared without a trace after 1912.
The official sources have dismissed both events as mass hysteria fueled by hoaxes and falsely interpreted natural phenomena.
John Keel in his UFOs: Operation Trojan Horse (1970) argues that the mystery airships were, in fact, not of extraterrestrial origin but rather, of the "ultraterrestrial", i.e. built by an very advanced civilization living on Earth alongside the humanity (though they might be humans, too, only far ahead of the mainstream science). He also ties the UFO sightings to many unexplained supernatural phenomena.
- Mystery airship. Wikipedia.