Carbon is arguably the most versatile element in the periodic table. It can form bonds to other elements in one-, two- and three-dimensions, allowing the formation of structurally and electronically diverse materials. Carbon allotropes, materials made only of carbon, were for long limited to diamond, graphite, fullerene, carbon nanotubes, and graphene. However, recently a series of zero- one- and two-dimensional carbon allotropes have been made. Until now, diamond is the only known three-dimensional carbon allotrope. Here we report the successful synthesis of a new 3D carbon allotrope, which we refer to as diamondiyne. The synthesis of diamondiyne is performed using inexpensive laboratory glassware. It is formed as a film at a liquid-liquid interface, and we show that the method is scalable in both the thickness and lateral area of the film. The received films are polycrystalline, and the crystal structure has been confirmed using transmission electron microscopy. Our results enrich the carbon allotrope family, enabling non-naturally occurring ones extending in full 3D space. New carbon allotropes have historically found widespread use in materials science, and we look forward to what applications might emerge for diamondiyne.