Hippasus was according to legend murdered by the Pythagoras for finding out that the square root of two was irrational. Hippasus was found face down in the mediteranian. He died of water inhalation's

Pythagoras drowned him.

Hippasus, However Pythagoras could not accept the existence of irrational numbers, because he believed that all numbers had perfect values. But he could not disprove Hippasus' "irrational numbers" and so Hippasus was thrown overboard and drowned.

Hippasus, a Pythagorean philosopher is believed to have proven the existence of numbers that are not rational. However, this is not quite the same as "known for". Many (most?) mathematicians would not know who Hippasus was or what he did.

Hippasus was a Greek mathematician of antiquity. He is credited for his proof that the square root of two is irrational, meaninng that it cannot be expressed as a ratio of two integers. The ancient Greeks were evidently not pleased with discovery, as legend has it that he was killed for his discovery! Physics guy