Theoretically yes — we already have the basic engineering principles. The main challenges are energy, material fatigue from seawater exposure, and the psychological toll of living without natural sunlight. A true city would require breakthroughs in materials science and energy generation, likely fusion power. Shallow reef stations are achievable within decades; deep trench cities would require centuries of development.

Shallow reef zones (20–50m) are most practical with near-future technology. Continental shelf depths (200m) are theoretically achievable with advanced pressure-resistant materials. Deep trench habitation (2,000m+) faces over 200 atmospheres of pressure, requiring entirely new engineering paradigms. Most serious proposals target the 200–4,000m range using buoyant spherical modules anchored to the seafloor.

There are three main approaches: sealed habitats with recycled air (like the ISS), surface-supplied air through pipes, or — the long-term speculative option — biological gill modification using advanced bioengineering. At shallow depths, algae cultivation inside domes could generate oxygen through photosynthesis, creating a partially self-sustaining ecosystem that also provides food.