Lenticular cloud over extinct volcano at sunset, Patagonia, Argentina (© David H. Collier/Getty Images)

【March 29 2013】When moist air flows over a mountain, it can produce cloud layers that stack up in these massive, stationary formations. The saucer-like shape of these lenticular clouds has caused many people to mistake them for UFOs. Explore breathtaking Gallery from the HuluTrip homepage over the past 5 years..