[pjc50]

It's essentially an old meme; the exact details had been lost, since the technology was not in continuous use and no written record survived, but the footnote on Wikipedia details its reconstruction. This great paper: https://pubs.geoscienceworld.org/msa/ammin/article/98/10/166...

Essentially it relied on volcanic ash from specific locations, and they forensically traced those locations.

[astroflask]

Right, that's the impression I got from reading. And it looks like we have regained that technology, which is what threw me off the remark. Will take a look at that paper, however I'm totally a stranger to most geo-something sciences, just a regular software engineer here (with quite a bit of scientific curiosity though).