Most bath sponges these days are artificial, but a reminder of when a sponge meant the dried up skeleton of a sea sponge. As long as 4,000 years ago, Greeks were free-diving for sponges in the Mediterranean Sea to sell for bathing. A sponge diver had a dangerous job, facing steep coastlines, sharks, and deep water. Special equipment gradually came into use, starting with a simple rake and advancing during the 1800s to a dive suit with a long tube connected to a boat air supply. The uses of sponges expanded to include cushion stuffing, agricultural fertilizer, scouring pads, and food additives. Modern sponge divers are often sponge researchers (spongologists) with scuba tanks, dive computers, or even submersibles. Research on sponges has allowed us to better understand their value to us. Because sponge cells can tell apart self from non-self, they have served as simple models for study of our immune system. Today, they are mostly used for their pharmacological properties; many sponges contain medicinal compounds.