The circular cloud was captured by an amateur cameraman above Mexico on February 11, and the video comments on YouTube suggest that it could be a hole punched in the sky by an alien craft.
© Metro.co.ukThe 'UFO' cloud hanging over Mexico
Some internet UFO enthusiasts have maintained that the cloud is absolute proof of a UFO visit, while others think it may be a porthole through which alien ships could enter the Earth's atmosphere.
Even the Met Office has been left baffled by the mystery cloud, or its causes, with spokeswoman Sarah Holland stating: "Aircraft exhausts could play a part in the freezing process but the precise cause is unknown."
Some believe that the phenomenon in Mexico can be explained as a result of a meteorite punching through the cloud layer. UFO expert Nick Pope adds "The similarity with the so-called Halo cloud seen recently over Russia is striking. One possibility is that a meteorite or fireball punched through the cloud.
"This could explain the strange sound reported and the fact that a crater was apparently found."
likely a "hole punch" cloud. A natural formation where lower altitude clouds push upwards into higher altitude clouds. Or perhaps caused by aircraft. See this one
[Link]
that I spotted 7 years ago.
See this
[Link]
for more examples
Modern science still knows very little about our atmosphere and its workings. Let's not jump to conclusions.