I bet mermaids are really great astronomers. I mean, their eyes must be super sensitive to light if they can see underwater, and there’s no place with clearer skies than the open ocean.
What if they found navigational equipment from sunken explorers’ ships and turned the old telescope designs and star charts into something more compatible with their habitat? Optics made of polished sea-glass, extraterrestrial communications systems based on the languages of different sea creatures, etc.
People on land talk about how they know as little about the oceans as they do about outer space, but merfolk already have the seas figured out.
So of course, what are they going to explore next? I think the answer is just as clear as it is awesome.