An article discussing a theoretical form of aluminum (via a certain crystalline arrangement) with a bulk density of only 0.61 grams per cubic centimeter… meaning it would float. The researchers behind this study haven’t actually produced any, so it’s not certain that it can be done on any sort of scale.

Ultra-Light Aluminum Eyed for Applications in Electronics, Medicine, Spaceflight

The research paper is paywalled, so I can’t tell if the researchers have predicted what the mechanical properties of the stuff would be. *If* it is anything like conventional aluminum, it would make for some interesting applications, especially if it has interesting thermal properties. But it’s metastable, which means that it *could* fall apart rather suddenly, potentially energetically. Quite likely what it means is that the low density aluminum would sit there and be well behaved until some external force came along – a temperature increase, water, oxygen, a whack with a hammer, foul language, a stern glance – and then the crystalline structure folds up like a university dean confronted with a pack of feral SJWs and the density goes back to normal.