This is what strikes me as notable: there are no Frontiers anymore (arguably there never were without massacring the existing inhabitants).
Space actually is one. And there are no (sentient) existing inhabitants.
IMO our current civilization has no way to account for, nor price, that reality - yet.
The oceans are our frontiers.
We know more about deep space than we do about deep water.
Additionally, an underwater civilization would make Humanity more resilient to some kinds of extinction events, if you care about that.
An underwater civilization would be much less resilient to a nuclear attack originating from Earth, than an extra-terrestial civilization would be.
Submarines and nuclear tipped torpedoes can strike any point on the sea floor with almost no warning.
Indeed! If we can survive the wrath of The Deep Ones, we can survive anything!
The oceans are all owned by various parties. They're use is governed by treaties and governments aggressively militarize development there (see how fishing boats are tracked and monitored).
They're very much not a frontier.
That covers the surface.
There are places on Earth that we have never been to. The deep ocean is incredibly hard to get to.
An extinction level asteroid hitting the Earth'll hose an underwater settlement just as sure as an aboveground one.
There are regularly scheduled flight from e.g. London and Oslo for ~$100 to Spitsbergen. Plants grow there and it's inhabited by native mammals, not just "a dozen penguins". Apart from the adjectives "snowy" and "cold" being used to describe it, it has little in common with Antarctica.
One of the advantages of a Mars colony is that the argument for it infringing upon any existing interests is much weaker than for anywhere on Earth. Of course, to some people, the idea that someone could just go somewhere and do a thing without anyone (who those people of course claim to speak on behalf of) being in a position to claim injury is disturbing in itself.