Helium

Helium is the second lightest and second most common element in the universe, but it is scarce on Earth because it is light enough to escape into space. Almost all of our supply comes from natural gas wells, where it builds up slowly from the radioactive decay of uranium and thorium underground. It was first spotted in 1868 as a strange yellow line in sunlight, decades before anyone found it on Earth.

Helium forms no ordinary compounds. Liquid helium boils at about −269 °C, colder than any other liquid, which makes it essential for cooling the magnets in MRI scanners. It also fills party and weather balloons, shields welds from air, and serves as a clean, inert gas in chipmaking.