i've been reading Bill Bryson's A Short History of  Nearly Everything

(great book, i highly recommend it), and he discusses the creation of

the first diving helmet, back in 1823.

     the designer was an englishman, Charles Deane, but the helmet

actually was not intended for diving.  it was intended as a helmet for firefighters.

    here's what Bryson has to say:

     [Deane's helmet] was intended not for diving but

     for firefighting.  It was called a "smoke helmet,"

     but being made of metal it was hot and cumber-

     some and, as Deane soon discovered, firefighters

     had no particular eagerness to enter burning

     structures in any form of attire, but most specially

     not in something that heated up like a kettle and

     made them clumsy into the bargain.  In an attempt

     to save his investment, Deane tried it underwater

     and found it was ideal for salvage work.

 A Brief History of Nearly Everything, p. 241 (2004 ed.)

                 Second-generation dive suit,  as developed by Augustus Siebe

    early on, these suits had a not-insignificant  problem.  That was that

should the air being pumped from the surface stop for some reason

(pump failure being the most common), the pressure differential created

would litterally "suck" the diver into the helmet -- the results were 100% fatal.

    Again, Bryson on the subject:

     Divers sometimes experienced a dreaded

     phenomenom known as "the squeeze."

     This occurred when the surface pumps failed,

     leading to a catastrophic loss of pressure

     in the suit.  The air would leave the suit

     with such violence that the hapless diver

     would, all too literally, be sucked up into

     the helmet and hosepipe.  When hauled

     to the surface, "all that is left in the suit

     are his bones and some rags of flesh,"

     the biologist J.B.S. Haldane wrote in 1947,

     adding, for the benefit of doubters,

     "This has happened."

A Brief History of Nearly Everything, p. 241 (2004 ed.)

    at some point, a reverse valve was invented that would shut off if the

pump at the surface failed.  That way, the air in the suit would not be

able to rush to the surface, taking the flesh of the diver with it.  The

reverse valve didn't solve everything, though... the diver was still

at depth with a very limited supply of air (whatever was in the suit

before the pump failed).

  those guys sure earned their pay, in my book.