The Crystal Cave of Giants, about 1000 feet below the surface in the Naica Mine, Chihuahua, Mexico, where lead, zinc, and silver are mined. The host rock is limestone, and the crystals are selenite (gypsum). See figure at left for scale.
More information: http://www.crystalinks.com/mexicocrystals.html
Uluru, or Ayers Rock, in Australia, which has always fascinated me. There is no way to judge scale in this photo, taken at sunset, but Uluru is 1,142 feet from base to top.
The Delaware River as seen from the walking bridge that connects Bulls Island State Park in New Jersey to Lumberville, Pennsylvania.
Image credit and copyright: Lew Fuller.
Silex Spring in Yellowstone National Park, Wyoming, is heated to steaming by the subterranean magma chamber called the Yellowstone Hotspot. The colors in the pool are caused by layers of bacteria that grow in the heated water. And of course that’s the Milky Way above it.
Image credit and copyright: Dave Lane