To rephrase the old saying of “what goes up must come down”, in the deep diving world we recognize what goes down must come up (at least for air-breathing mammals). But as with many straightforward concepts we realize that the devil is in the details. An examination of history can illuminate how much of the detailed knowledge we so often easily take for granted today was, more often than not, hard-earned over a very many years. Such has been the case over time with the struggle to elucidate the physiological and pathological effects on humans who choose to operate in pressures higher than that of sea level.
Diving did not start with the ability to provide a constant supply air to subsurface human activities—breath-hold diving was being practiced many hundreds, if not thousands, of years before this. People dove—and sometimes still dive today—in this manner for food, sponges, pearls, salvage, and military purposes, to name but a few reasons. Reports suggest that by 500 BC, ancient Greeks and Persians were diving for valuables lost in sunken ships. Evidence exists to indicate that along the coasts of Japan and Korea, breath-hold divers were starting to harvest seaweed and shellfish down to depths of ~20 m over 2000 years ago. And so began the saga of the famous female divers, well-known as “Ama”, from these Asian countries. Thousands of (mostly) female Japanese and Korean breath-hold divers are still today using techniques largely unchanged from the earliest years. It is of interest to note that the more recent history of breath-hold diving records performances to great depths. Many world-class breath-hold divers have exceeded 100 m in depth. Perhaps the biggest physiological challenge for breath-hold divers going to extreme depths is the drastic changes in ambient pressure encountered in a deep dive. Hydrostatic pressure acting on the body increases by approximately 1 atmosphere (atm) for every 10 m of depth, so at ~30 m, for example, the ambient pressure is the sum of the atmospheric pressure (1 atm) plus the hydrostatic pressure (3 atm). Thus, Boyle’s Law tells us that the gas volume in a breath-hold diver’s lungs will be compressed to one-fourth of the volume it had at the surface - and consequently alveolar gas pressure will be increased four times, as well.
The Italian mathematician Giovanni Borelli is credited with being the first, in 1680, to develop (in concept) a system to free humans from surface air sources. The diver wore an enormous watertight bag, filled with air, around his head - with the bag having a small port for vision and a rudimentary system for directing exhaled air out of the bag. However, this model failed - on descent the pressure on the air bag and diver’s body never differed. Of course, the magnificently practical modern version of his concept that uses compressed air to maintain inflow of air to the diver is the self-contained underwater breathing apparatus (SCUBA) developed between the mid-1920s and 1943 – 1943 being the year that Jacques Cousteau and Emil Gagnan introduced for the first time a system that would be easily recognized today as a SCUBA unit.
Since the technical requirements of self-contained breathing apparatus for use at depth was beyond the applied scientific abilities of the middle-ages, other sources of breathable air were investigated for divers during this era. A practical solution was found in the development of the diving bell, a rigid vessel which was in regular use for mineral exploration, submarine construction, and salvage by the mid-1500s as a structure within which to perform work that was suspended from the surface by a rope or chain (Figure 1).
Figure 1- Halley’s diving bell
The major limiting factor with the diving bell was that the only available air for breathing was that which was trapped under the dome of the bell at the time of submersion, which of course impacted how long divers had breathable air at their disposal. As carbon dioxide accumulated in the non-circulating air of the bell, the working environment would become intolerable.
With the coming of steam engines in the late 1700s, it became possible to efficiently force a continuous supply of fresh air into diving bells. This, and other technological advances, will be discussed in subsequent articles.