Antarctica's "Doomsday Glacier" is about to lose an ice shelf that protects it from warming ocean waters, which could have devastating long-term consequences for hundreds of millions of people affected by rising sea levels.

The already-melting Thwaites Glacier is nicknamed the "Doomsday Glacier" because if it totally collapses, global sea levels would rise by 26 inches (65 centimeters) and flood coastal communities. Rising sea levels are already threatening the residents of major cities in the U.S. and throughout the world, including New York, Boston, San Francisco and Miami.