Antelope Canyon, Arizona- After you squeeze into the canyon you're met with a metal ladder to help you get to the bottom. Before the ladder was installed travelers had to rappel to the bottom, and there wouldn't be any way to get out of the canyon at the opposite end without a ladder. Photo by Travis Burke.
"Mammatus clouds are most often associated with the anvil cloud and severe thunderstorms. They often extend from the base of a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds. When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm or perhaps even a tornadic storm."