Three quarters of the Earth’s surface is covered in water. So if you wanted to hide your advanced craft, where better to choose that to head for the bottom of an ocean?
In the Pacific Ocean, there is an anomaly off the coast of California, known as the Malibu Anomaly.
There is also another anomaly in the Baltic Sea that has drawn interest from divers and Ufologists alike.
Whilst in the east, the Russian military are said to have had an encouter with strange creatures in Lake Baikal.
Of course the reason so much under the water remains unexplored is the sheer difficulty of exploring at great depths. Not only is it difficult, it is very expensive. That freezes out most independent researchers, so only those with rich benefactors, or scientists with huge research grants can actually afford to get out there. Which is another good reason to hide your advanced craft deep in the water.
So if you wanted to hide a highly advanced craft, or even build a hidden base, where would you put it?