Underwater settlements were a staple of the 60's and 70's literature, but they seems to have fell out of favor since, despite being more feasible than outer space exploration. The technology exists in my opinion, it's a matter of money and maybe environmental regulations / legal status of international waters.
Yet all we ever had are a few science labs, and some novelty hotels with observation bays just under the surface.
Would it be possible to have a POD during the XXth century leading to the establishment of a proper permanent underwater colony? If so, how big?
Egomaniacal founder and mutated abominations optional.