Some time ago, I wrote about a fascinating 1975 experiment in which the relationship between quantum mechanics and gravity was tested.  The experiment was made possible by the new — at the time — technique of neutron interferometry, in which the wave properties of matter are exploited to test quantum effects.  The experiment confirmed a very odd theoretical prediction: that the particle-like aspects and the wave-like aspects of matter are effected differently by a gravitational field.  What, exactly, this implies about the nature of the universe is still, as far as I know, unclear!

Not long after this work was done, further investigations using essentially the same neutron interferometer tested, and confirmed, what I consider to be one of the strangest aspects  of quantum mechanics, and another powerful illustration of how the universe obeys really different rules on the atomic scale.