11 mysteries attributed to aliens

Crop Circles These detailed, circular patterns cut into wheat- or corn-fields started appearing regularly in the United Kingdom in the 1970s, and are believed to be messages from aliens (or some very bored humans.)

Cow Mutilation Since the 1960s, US ranchers have occasionally found that their cattle have been assaulted in the night. Sometimes the animals' ears appear to have been cut off with a laser. (Daniel Schwen)

Nuclear Missile Failure In 1967, a UFO hovered over the Malmstrom Air Force Base in Montana, shutting down all missiles at the site, <a href="http://www.foxnews.com/scitech/2010/09/23/aliens-monitoring-nukes-worry-ex-air-force-officers/">according to numerous insider accounts</a>. Allegedly, UFOs have disabled nuclear missile systems on several military bases around the globe. (U.S. Navy)

Machu Picchu Some believe that aliens must have built Peru's Machu Picchu, as it seems too sophisticated for humans to have created in the 11th century. (Fabricio Guzmán)

Easter Island Heads Between 1100 and 1680, aliens were apparently stranded on the island and amused themselves by carving these giant statues.

The Moon Kindly aliens, it's said, created the Moon around 4.5 billion years ago so that Earth would have tides; a few stayed behind in caves under the Moon's surface for surveillance. (T.A. Rector, I.P. Dell'Antonio, NOAO, AURA, NSF)

Uffington White Horse Three thousand years ago, this giant horse figure was etched into Earth and filled with ground-up white chalk so that aliens could see it form space. (USGS, NASA)

Stonehenge It's rumored that aliens loaned humans their superior technology and knowledge of the solar system to make this structure around 2500 BCE. (Matthew Brennan)

Cappadocia Some believe that aliens built Cappadocia's system of underground tunnels around 1200 BCE and interpret UFO sightings in the area as proof of this theory. (Brocken Inaglory)

Pyramids Whoever built the Pyramids had access to some amazing masonry tools, and had a sophisticated understanding of mathematics of 2630 BCE. Some theorize that aliens must have lent a hand. ((Rob Alkmaar))

Nazca Lines These enormous line drawings were built in 400 CE, some say to mark an ancient alien airfield - and potentially a very confusing one, given that many of the lines depict animals or other designs. (Maria Reiche)