Helium
From RationalWiki
Helium is a noble gas, the lightest of the noble gases. It holds court in stars and kid's balloons.
Helium is the second lightest element, heavier only than hydrogen. Due to its non-reactive nature, especially compared to the extremely flammable hydrogen, helium is the preferred element whenever a light gas is needed. This is why it fills kiddy balloons.
Due to its odd displacement qualities, it has peculiar effects on human vocal cords at normal pressures. However, at extremely high pressures (like near the bottom of the sea), it's been found to be extremely useful. Deep-sea divers breathing pressurized air have problems with bubbles of nitrogen working into their joints, which tend to painfully come out if they depressurize too quickly, a condition called the "bends". It's been found that an oxygen/helium mixture does not cause this problem.
Since helium is so light and does not bind with other elements, no helium remain on the Earth from the initial formation of the planet. All existing helium, therefore, is relatively new, manufactured by the alpha decay of radioactive elements deep inside the planet.
