“I have this one little saying, when things get too heavy just call me helium, the lightest known gas to man.” –Jimi Hendrix
The second lightest and second most abundant element in the Universe, helium, is incredibly rare on Earth. Practically none of the helium that Earth was formed with still exists, since was too easy for it to escape from our tenuously held atmosphere, unlike the gas giant worlds. But deep underground, in the heavy-element-rich interiors of the Earth, new helium is continuously produced.
The heaviest unstable elements, like thorium, uranium and radium, are particles that undergo alpha decay on timescales of hundreds of millions to billions of years. They give rise to massive pockets of helium, enabling us to extract it for scientific, medical and more frivolous purposes. But if we waste this cheap, abundant and easy-to-obtain material now, humanity doesn’t have hundreds of millions of years to wait for it to replenish itself.