When the tide at sea rises, the water in wells falls, and vice versa – something known since around 100 BC and explained only in 1940
The Greek philosopher Poseidonios, who lived between 135 and 51 BC, discovered this effect. He noticed that, when a spring at the temple of Heracleium in modernday Cadiz in Spain was low, the tide in the nearby Atlantic was high, and vice versa. The explanation is that the gravity of the Moon, the principal cause of the tides, not only makes the ocean bulge upwards but it does so to the rock as well. The waterlogged rock in which a well sits is like a wet sponge. It sucks water out of the well when the sponge is stretched upwards (high tide) and squeezes water back into the well when the sponge is released (low tide).
The Moon doesn’t just cause tides, it causes the Earth itself to bulge, something that was seen (but not understood) in the unusual behaviour of a well