Michelson–Morley: the failure that led to relativity
They looked for an ether wind and found the constancy of the speed of light.
In the nineteenth century light was thought to travel through the ether, a medium filling space. The Earth moves through the ether, so the speed of light ought to depend on direction.
In 1887 Albert Michelson and Edward Morley assembled an interferometer on a stone slab floating in mercury, so the instrument could be turned smoothly without vibration. Its precision was enough to detect an effect tens of times weaker than the one expected.
The result was null. However they rotated the apparatus and whatever time of year they measured, the speed of light was the same.
That negative result became one of the most productive in the history of physics: it went into the foundations of special relativity, where the constancy of the speed of light is not a consequence but a starting postulate.