Step one: stow lead ingots on a Roman freighter.
Step two: sink the freighter off Sardinia.
Step three: wait two-thousand years.
Step four: profit!
Apparently, this economic model also works for WWI German warships.
Who knew radioactive metal was such a widespread problem? It's nice to be reminded once more of the unintended consequences of nuclear testing--especially given how insane the intended consequences already were.