Pythagoras and his students, through the study of musical harmonics, believed that the world was based on natural numbers (integers) or ratios of integers. This was almost a sacred belief for them. Supposedly they discovered the square root of two through the study of triangles; it is an irrational number which cannot be expressed as the ratio of two natural numbers. Legend has it, they tried to keep this number secret because it undermined and discredited the premise of their sacred mathematics.
“It's a stormy day on the sea off the coast of Greece. The date is around 520 BC. Fighting for his life, a man is heaved over the side of a boat and dropped into the open water to die. His name is Hippasus of Metapontum. His crime? Telling the world a mathematical secret. A secret only known to the school of Pythagoras. The secret of the dangerous ratio – the square root of two.”
https://nrich.maths.org/2671