Ceramic-and-mortar seals on the garum jars were corroded by seawater or removed by the occasional curious octopus, the archaeologists report, but traces of the fish sauce remain inside.
So it's probably just about as edible as it was in the time of the Caesars. In case you're interested, here's a recipe from Gargilius Martialis's De medicina et de virtute herbarum:
Use fatty fish, for example, sardines, and a well-sealed (pitched) container with a 26-35 quart capacity. Add dried, aromatic herbs possessing a strong flavor, such as dill, coriander, fennel, celery, mint, oregano, and others, making a layer on the bottom of the container; then put down a layer of fish (if small, leave them whole, if large, use pieces) and over this, add a layer of salt two fingers high. Repeat these layers until the container is filled. Let it rest for seven days in the sun. Then mix the sauce daily for 20 days. After that, it becomes a liquid.
(Reprinted from A Taste of Ancient Rome)
For this and other ancient Roman recipes (some of which look perfectly nontoxic), see Nova Online.
There were several types of garum that were made, from the cheap variety to the really good stuff. Still, fish is fish.
The best part of Latin class! Bwaha!
My brother made a batch in his fifth year of high school and taped it. Now my Latin teacher shows it every year, and he still has the bottle (from like, 5 years ago). He won't let us taste it anymore, though, because there's some kind of unknown sediment in the bottle.
I think we might go ahead and make some more this year.