Elsewhere on this website we have asserted that the reason we cannot observe civilizations on other planets and moons is that the residents dwell below the surface. This does not always necessarily entail the celestial bodies being hollow, sometimes it just means that the beings and their cities are subterranean.
According to reports posted this week on News.com, Nexus Newsfeed and Unexplained Mysteries.com, our earthly scientists seem to be catching on:

NEWS HEADING
Planetary scientist Alan Stern has suggested that some civilizations may live below a planet's surface.
            Speaking at a recent meeting of the American Astronomical Society's Division for Planetary Sciences, Stern, who is the principal investigator of the New Horizons Pluto mission, put forward one possible explanation for why we have yet to encounter intelligent extraterrestrials.

Image credit: https://nexusnewsfeed.com/article/science-futures/some-civilizations-may-live-below-a-planet-s-surface/

            The key, he argues, could be that not every alien civilization actually lives on its planet's surface. Recent findings have suggested that planets with habitable subterranean oceans could be quite common - perhaps even more common than worlds with habitable surface conditions.
THIS IS WHERE THEY BURY THEIR HEADS, THOUGH, LIMITING THEIR POSSIBILITIES TO AQUATIC LIFEFORMS:
            There is a chance that alien life forms - even intelligent ones - could evolve and form complex societies entirely underwater without ever venturing to the surface. "Water worlds are naturally cut off from communication by their interior nature below a thick roof of ice or rock and ice, therefore do not easily reveal themselves," said Stern.
            Some scientists however, such as Dr Fergus Simpson from the University of Barcelona, have questioned whether such an environment could ever actually support intelligent life forms. "The main reason I'm sceptical about there being lots of intelligent underwater alien species is that I think it's harder for aquatic creatures like dolphins to use tools or build a fire," he said.

[DR SIMPSON NOT ONLY THINKS INSIDE THE SQUARE - HIS SQUARE IS A POOL OF WATER AND HE IS RIGHT AT THE BOTTOM OF IT.]