In March 2014, a video titled simply "UFO caught on Camera" was uploaded to the "Most Mystery Videos" channel on YouTube. The video, apparently shot from the deck of a small fishing boat at sea (claimed in the description to be off the coast of Galicia, an autonomous community in the northwest of Spain), starts with two fighter jets flying over the fishing boat at considerable speed. The camera then captures a UFO, looking like the traditional two upturned plates stuck together, appearing from the right and plunging into the sea. As the fishermen point and shout excitedly at what they have just seen, what appears to be a military helicopter enters the frame and, allegedly, orders the fishermen to leave the area immediately (apologies: I don't speak Spanish and so am relying on what other people have said is being said on the video).
Despite being uploaded in 2014, the footage was apparently captured in 2009, which lines up with the badly pixellated nature of the footage. While the YouTube channel mentioned above appears to have removed it, and possibly several other videos as well as it now only has 11 videos visible, the oldest uploaded some time in 2016, it has been uploaded to several other UFO and paranormal mystery channels, where over the years it has garnered 100,000s of views and a great many comments, the majority of which excitedly state that it looks so "real and authentic" that it "must be real"... despite all the evidence pointing to the contrary.
Yes, I am afraid this footage is fake, although it was not a deliberate hoax. The footage was in fact made as part of the viral marketing for a film called El OVNI Gallego - The Galician UFO. This film was made in 2009 to promote the website/web portal Terra Spain. In the video posted above, you can even see the yellow "Terra" logo in the bottom right corner.
In what is probably a great moment of irony, El OVNI Gallego was made as a tribute to Orson Welles' 1938 War of the Worlds radio broadcast. It was never made to trick people into thinking it was real, and the filmmakers later released a video showing how they made this original 43-second video.
So in trying to make something to celebrate the 71st anniversary of the original War of the Worlds, the filmmakers managed to make something that was thought (and apparently still is by some) to be actual footage of a UFO being chased by the military and submerging itself in the Mediterranean Sea.
Sources:
"Fishermen films military jets chasing UFOs over Mediterranean Sea" (FisheryNation)
"UFO Caught on Camera by Spanish Fishermen: Facts" (Hoax or Fact)
"El desenlace del caso del OVNI gallego: apareca la tercera cápsula" (Mundo insólito, via Wayback Machine)