Octaoxygen is an allotrope of oxygen consisting of eight oxygen atoms. As the pressure of oxygen at room temperature is increased through 10 gigapascals (1,500,000 psi), it undergoes a dramatic phase transition to a different allotrope. Its volume decreases significantly, and it changes color from sky-blue to deep red. This ε-phase was discovered in 1979, but the structure has been unclear. Based on infrared spectroscopy, researchers assumed in 1999 that this phase consists of O4 molecules in a crystal lattice. However, in 2006, it was shown by X-ray crystallography that this stable phase known as ε oxygen or red oxygen is in fact O8. No one predicted the structure theoretically: a rhomboid O8 cluster consisting of four O2 molecules.