This is kinda interested and sort of strange, and that it wasn't discovered earlier.
"May 2014 The depth from the surface of the water to the top of the structure is is 450 meters or 1476 feet and to the bottom of the entrance is 700 meters or 2296 feet. Below are an additional nine images taken from due south looking north, going around and ending due north looking south towards Los Angeles and Santa Monica".
Source: Third Phase of Moon UFO
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UFO base cave found off of Malibu, California coast ? (video)