Far below sea level, they contain some of the harshest environments on Earth, with no oxygen and no light.

VOA: special.2010.11.10

This makes it more difficult for the body to effectively use the same amount of oxygen as it would on the ground.

VOA: special.2009.07.14

Engineers had to boil off the remaining oxygen by turning on a heater in the tank.

VOA: special.2009.07.22

It carried oxygen,water, electricity and food for two men for two days on the moon's surface.

VOA: special.2009.07.22

He said the great machines that produced the atmosphere on the planet had stopped producing oxygen.

VOA: special.2010.07.31

Doctors say the damage brought on by these conditions eventually interrupts the natural exchange of oxygen and carbon dioxide in the lungs.

VOA: standard.2010.06.21