The first descents

By 2120, what had begun as ad-hoc shelters at recreational and technical depth had been replaced, in the principal coastal regions, by pressurised cylinder habitats anchored to the continental shelf between roughly fifty and two hundred metres. The transition was not a single act of policy. It was, by all accounts, a slow accumulation of failures of the alternative.

Where the early habitats sat

In the band between fifty and two hundred metres. Shallow enough that resupply from the surface was practical; deep enough to be below the storm layer; engineerable with materials that could be sourced from the surface industries that remained. The shallowest sites — the upper-rib anchorages — were technical-diving depth, which made transit comparatively cheap. Deeper sites were saturated.

Why not deeper

ADS technology of the period was capable of perhaps four hundred metres for short sorties. Permanent pressurised habitation past three hundred metres awaited a generation of saturation engineering that was developed over the next twenty years. The first descents settled where the existing technique could keep people alive.

What this period did not yet include

Contact with the Tyrian Court, or any other Pelagine polity. The bathyal was not yet a place humans visited; the survey was a coastal survey. The deeper records begin only after 2150.