I recently found out that 10 000 Muons are hitting every square meter of Earth a minute. They are created by high energy collisions in our upper atomsphere that then fly at nearly the speed of light down to earth and survive even though they have a short half-life because of relativistic effects slowing down their decay.

It's crazy to think that natural radiation can include strange unstable sub-atomic particles, not just alpha, beta and gamma radiation.