The Pentagon's refusal to answer questions on underwater UFOs at its public hearing could mean the truth about the infamous Nimitz Tic Tac encounter is revealed in SECRET, an expert said.

In the first hearing to discuss unidentified aerial phenomena - or UAPs - since the 60s, held on Tuesday, defense officials admitted there had been 400 sightings and 11 near miss collisions with UFOs.

However they refused to answer when asked about underwater UAPs - also known as USOs - in the open session - and referred the question to their secret behind-closed-doors session, which was held after the public hearing.

Alejandro Rojas, a UFO researcher from the Scientific Coalition for UAP studies, told The Sun this might be because they want to reveal a top secret technology or something even more "mysterious".

He also pointed out that would mean any answers on the infamous Nimitz UFO encounter - where pilots chased a craft they described as looking like a "40ft Tic Tac" above the Pacific ocean - could be kept secret as the incident involved a UAP both in the water and the air.

"I think that the most interesting part of the hearing was when underwater UFOs were brought up," Rojas said.

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