Diver's Helmet

A diving helmet is a rigid head enclosure with a breathing gas supply used in underwater diving. They are worn mainly by professional divers engaged in surface-supplied diving, though some models can be used with scuba equipment. The upper part of the helmet, known colloquially as the hat or bonnet, may be secured to the diver or diving suit by a lower part, known as a neck dam or corselet, depending on the construction. Quoted from Wikipedia.