You can pick up a synthetic sponge for just a few dollars. But a natural sea sponge, hand-harvested from the Mediterranean seafloor, could set you back $30. Harvesting these sponges is dangerous, and the risks don’t guarantee big rewards. Divers, or “the sponge men” as they’re called in Kalymnos, Greece, can find some of the most valuable sponges in the Mediterranean Sea. But these days, they're not so easy to come by. So what happened to the once-booming sea sponge population here? And why are these sponges so expensive?