What observation lead to The theory of relativity?

The theory of relativity, developed by Albert Einstein, was largely influenced by observations of the behavior of light and the inconsistencies in Newtonian mechanics at high speeds. One key observation was the constancy of the speed of light, regardless of the observer's motion, as demonstrated in the Michelson-Morley experiment. This led Einstein to propose that space and time are interconnected in a four-dimensional spacetime framework, fundamentally altering our understanding of gravity and motion.