Boron is known for its structural complexity, unusual type of bonding, and for a rich variety of compounds that it forms. Until recently the existence of only two boron modifications obtained at ambient pressure, - and -rhombohedral boron, seemed to be proven beyond doubt. The goal of the presented research is a systematic investigation of boron's possible phase transformations under high pressure and temperature conditions. The major outcome of the study is the discovery of a new boron phase with 28 atoms in an orthorhombic unit cell. B28 exhibits striking physical properties. Its crystals...