A man went two years before noticing something incredible living inside his fish tank.

He noticed coral colonies missing and decided to clean out the tank. He discovered a massive bristle worm, also referred to as a Polychaete living inside. Once he realized what it was, he said he only saw it three times over the course of the year.

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“It hides in the rocks, and only comes out at night, impossible to catch without taking everything out the tank,” the uploader wrote.

He said it measured about 1.2 meters in length.

(h/t Gizmodo)