"Tritium (/ˈtrɪtiəm/ or /ˈtrɪʃiəm/; symbol T or 3H, also known as hydrogen-3) is a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of protium (by far the most abundant hydrogen isotope) contains one proton and no neutrons. Naturally occurring tritium is extremely rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays."
(My emphasis - more @ the URL)
Naturally occurring tritium is extremely rare on Earth - but it is found here, from natural reactions.