The simplicity of the diving bell actually makes it believable that Alexander the Great may have used an early version, as legend suggests. Next time you're in the kitchen get a cup, fill up your sink and push the open end of the cup straight down into the water. You'll feel some resistance, but as long as you keep the cup straight up and down there will be a pocket of air inside the cup. This is the concept behind the diving bell.
The bells were cast in all kinds of shapes made from all kinds of materials. There were wooden bells that weren't shaped like a bell at all, but barrel-shaped. Cast iron bells were square or round, bell-shaped or conical. Some looked like whisky bottles while others resembled inverted wine glasses. The thing that each had in common was that they were all very heavy. This is why iron was commonly used. Lighter materials were weighted down and balanced with ballasts and weights. The reason it needed to be so heavy was because of the resistance you feel when you push the cup into the water.
The force of the water pushes the air up, compressing it as water enters the bottom of the open bell. While there is some water in the bottom of the bell, the bulk of it remains packed with breathable air. But there was one key limitation to this -- the bell could only go so deep and still have a usable pocket of air. A 10-foot tall (3-meter) diving bell that dove to 325 feet (100 meters) would only leave about 11 inches (30 centimeters) of air. For a while, divers simply tried shallow waters and ascended when the air was used up.
There was also the matter of decompression sickness, or the bends. This is when you ascend to the surface too rapidly after a deep dive. As you dive, pressure on your body increases, causing more nitrogen and oxygen to dissolve in your blood. Most of the oxygen is consumed by tissue, the nitrogen remains. This dissolved nitrogen is what causes the bends. If you ascend too quickly, the nitrogen leaves your blood too fast and forms bubbles. These bubbles block tiny blood vessels and can lead to strokes, heart attacks, ruptured blood vessels in the lungs and joint pain.
Divers endured these limitations while continuing to use versions of the diving bell during the Renaissance and into the 16th century. It would take some key innovators in the late 1600s and into the 1700s to improve the limitations of the diving bell.