On spatial asymmetries

March 2024

Consider these two lists:

That's right: twice as many people have walked on the moon than have dived under 300 m of water, despite the distances being a million times apart.

Horizontally, 300 m is barely the length of a typical city block; you could walk that distance to the corner bakery and back without breaking a sweat. Looking upward, 300 m is the height of a typical skyscraper; quite a workout if you took the stairs, but a pleasant afternoon hike if the gradient was spread over the more gentle slope of a hill. Beneath the ocean's surface, however, lies the titular asymmetry: a 300 m depth might as well be on the moon for the rest of us.

I find this natural asymmetry, and the tales of divers who dared confront it, quite fascinating. The crux of it, of course, lies in the density of seawater, which is 840 times that of air. It follows that the ambient pressure increases far quicker under water than it decreases toward space. The Kármán line, which is conventionally where space begins, is a distant 100,000 m above us; at that altitude, the difference with sea-level air pressure is nearly one bar. In comparison, a diver only needs to swim 10 m down to find the same pressure gradient.

This asymmetry is also why it is a far taller engineering challenge to pressure-proof a deep-sea vehicle than a space vehicle; the latter only needs to resist one bar of (negative) pressure gradient, whereas the former must resist one bar for every 10 m travelled below the surface. At 300 m of depth, the difference in pressure with the surface is a crushing 30 bars, which explains why submarines generally don't dive deeper than that. Furthermore, at that depth, complete darkness reigns; the water temperature is just above freezing; and gas loading into the tissues commit divers to very long decompression obligations before returning to the surface. Such an environment is as alien and hostile to human life as it gets on Earth.

It should now be obvious why a dive to 300 m is more akin to, if not more challenging than, an extravehicular exploration of space, compared to any other form of exploration of our planet. Just how the pressure manifests itself to divers at depth is another rabbit hole that I find fascinating in its own right. I explored that topic in another blog post.

Notes

¹ Including SCUBA and rebreather dives, and excluding saturation dives using a diving bell, simulated dives in a dry hyperbaric chamber, and Pascal Bernabé's 2005 rebreather dive to 330 m (1,080 ft) which was never officially verified.