The Mendenhall Glacier is a tongue of ice stretching 12 miles from the Juneau Icefield to Mendenhall Lake. At its widest point, the glacier is more than a half mile wide, with ice 300 to 1,800 feet deep. Rising global temperatures have caused the glacier to start melting—it has receded by about two miles since 1958. Water has carved caves into the interior, creating surreal, turquoise-toned worlds...