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@Merlin said, the Earth's oceans are vast. Just travelling from San Diego to Oahu, Hawaii you get a sense of how big the Pacific ocean is. My destroyer was something like 4o feet wide and 500 feet long and understand how small that is when thinking you are a 1000 miles or more in any direction from any land. When shyte happens on your boat, it's likely that no one can reach you in time if the ship goes down. The Indian Ocean may even be more massive than the Pacific. The World's oceans are massive and an underwater Alien base would be very hard to detect
Yet, if an intergalactic travelling species exists, why would they expend resources and incomprehensible amount of time to come see us? We are nothing interesting to them if they know ho do this. Carl Sagan called the distance radio waves have travelled since the first broadcast in the early 20th century, the "Pale Blue Dot." It's been a little more than 100 years ago and so the radius is more or less 100 light years away from Earth and this is the only evidence of intelligent (sorta) life existing here to a searching Alien species.
"The Milky Way stretches between 100,000 and 180,000 light-years across, depending on where you measure, which means a signal broadcast from one side of the galaxy would take 100,000 years or more to reach the other side. "
per a Popular Mechanics article found here
https://www.popularmechanics.com/space/news/a27934/galaxy-map-human-radio-broadcasts/
So if an Alien species could hide a base in our oceans and never be found with current human technology, what would be so special about us in the observable Universe, composed of 100 billion galaxies the size of our Milky Way galaxy, to come seek us out?
There are mysteries and underwater creatures unknown to us in our vast Oceans, but I don't believe the Earth has ever been visited by intelligent life with the capability of interstellar/ intergalactic travel. As Carl Sagan says, "In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves."
@CGI_Ram
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