File:Ama At Mikimoto Pearl Island.jpg
The word ama literally means "sea person". Until the 1960s, they dived wearing only a loincloth, and even now they dive without scuba gear or air tanks. It's supposed that the majority of ama are women because women naturally have more body fat, which ensures that they can stay warmer in colder water.
Here is a link to a recent article in the Guardian.