Radioisotopes decay; which isotopes are you thinking of that have half-lives of millions of years and would be found in detectable quantities? (I was going to

Radioisotopes decay; which isotopes are you thinking of that have half-lives of millions of years and would be found in detectable quantities? (I was going to raise the same point, thinking of low-background steel and the effects of nuclear testing on our atmosphere, but after some more reading it appears that most of those isotopes decay over shorter periods or weren't sufficiently elevated to be discernible from the background, if it were being examined in the far future.)