so, here's a little blurb by nancy harmon jenkins (from the essential mediterranean) on garum:

The oddest product that Greeks and Romans produced from salt, at least to modern tastes, was the thick syrupy liquid called garum, made from various parts of fish (some author say the intestines, others from small whole fish, still others from a combination of the two--and probably all of these methods were used at one time or another). The fish, layered in terra-cotta jars with lots of salt, sometimes with aromatic herbs added, was left in the sun to fermenet to a rich and heady brew.