The big thing to understand is that you gain a whole atmosphere of pressure every 10 meters underwater you go. So at 100 meters you are experiencing 11 atmospheres of pressure (over 150 PSI). It's a wonder humans can go there at all without a vessel!
It's not really about the distance; it's about how drastically conditions change in 100m depth of water -- to the point that very specialized equipment is necessary for humans to survive, and if we're not careful on the way up it's gonna be a Very Bad Time possibly (or even likely) resulting in death.
For sure. This got me thinking about how humans need special equipment to survive long even 1m below the surface, and the problems just get worse with depth. Wikipedia has nice coverage of the way we've iterated deeper and deeper: https://en.wikipedia.org/wiki/History_of_underwater_diving
> DCS [Decompression Sickness] is best known as a diving disorder that affects divers having breathed gas that is at a higher pressure than the surface pressure, owing to the pressure of the surrounding water. The risk of DCS increases when diving for extended periods or at greater depth, without ascending gradually and making the decompression stops needed to slowly reduce the excess pressure of inert gases dissolved in the body.
> Decompression time can be significantly shortened by breathing mixtures containing much less inert gas during the decompression phase of the dive (or pure oxygen at stops in 6 metres (20 ft) of water or less). The reason is that the inert gas outgases at a rate proportional to the difference between the partial pressure of inert gas in the diver's body and its partial pressure in the breathing gas; whereas the likelihood of bubble formation depends on the difference between the inert gas partial pressure in the diver's body and the ambient pressure. Reduction in decompression requirements can also be gained by breathing a nitrox mix during the dive, since less nitrogen will be taken into the body than during the same dive done on air. [85]