What is special is that it was chemically refined as lead in ancient times. Lead-210 is in the natural Uranium decay chain: https://en.wikipedia.org/wiki/Decay

What is special is that it was chemically refined as lead in ancient times. Lead-210 is in the natural Uranium decay chain: https://en.wikipedia.org/wiki/Decay_chain#Uranium_series https://en.wikipedia.org/wiki/Decay_chain#Uranium_series The uranium that feeds the chain has a half-life of ~4.5 billion years, so a handful of good 'ol dirt generally has the entire chain's worth of isotopes present. Refining the lead out of ore concentrates almost-pure lead (with all of its isotopes). Lead-210 has a 22-year half-life, so the lead-210 in Roman lead has almost entirely decayed away at the present day. Being underwater is also special. Cosmic rays strike nuclei on Earth's surface all the time, sometimes transmuting them into other, potentially-radioactive isotopes. They aren't of safety concern, but they are readily observed in low-background physics experiments. "Cosmogenic activation" is a serious problem -- the best facilities these days re-refine copper, from which the experiments are built, deep underground in order to remove cosmogenics. Similarly, detectors are transported by boat in order to prevent the activation that happens via air travel. Ancient Roman lead avoids both problems. It is old, removing the 210Pb, and has low cosmogenic activation.