Scientists still have to find irrefutable evidence proving the existence of UFOs. Hundreds of new reports on UFOs appear everyday. One percent of those reports remain a mystery unraveled though 99% of them could be explained by natural or man-made factors.

Let us assume that aliens exist and visit Earth on a regular basis. What would be the best place for them to build a secret base? Seabed would be the best option. The oceans cover two-thirds of the planet. The ocean remains virtually unexplored. We have a much clearer picture of processes that occur on the Moon, our knowledge of things that happen deep under the surface of the sea is scarce. As it turns out, reports about certain mysterious undersea incidents arrive steadily. Those incidents are reported by naval officers who normally have good technical expertise and composure. The Navy officers are subject to annual medical examinations...

long article, converted to .pdf if you want to read it all

[Midnightblueowl]

Yes, that is very possible. However, I can think of another option, shown in 'The X Files'. In that, they hide under the ice in Antartica, and the government's hide them from the public. Maybe they also cloak on ground and in the air. Or maybe they just don't land! There is countless options, if they exist at all!