|Heavy-water ice cubes sink in ordinary water (left).|
Here is the source. They don't explain (as far as I can see) what is happening here. Both glasses contain cold water, and the glass on the right has an ordinary ice cube floating in it, just as you would expect.
But the ice cube in the left glass is frozen 'heavy water' - deuterium oxide. As you can see, it is heavy.
p.s. If I'm right, that was an expensive photograph!