I remember a tuna boat going straight down in the ocean. The theory was that a submarine got caught in the netting. No idea which country.
A few decades ago, a cousin of a close friend was moving a large expensive sailboat from the East Coast (NYC area? the NE in any case) to Bermuda. Cousin was a very experienced, capable ocean sailboat captain.
In the middle of the night, the boat hit something mid ocean and went down fairly fast. Cousin barely had time to wake up, get into a lifeboat and hit an emergency beacon. He was picked up in mid ocean.
The cousin’s theory was either a semi-submerged floating container (think full of Styrofoam packing), or a submarine.