  In a semimetal, both electron and hole carriers contribute to the density of
states at the Fermi level. The small band overlaps and multi-band effects give
rise to many novel electronic properties, such as relativistic Dirac fermions
with linear dispersion, titanic magnetoresistance and unconventional
superconductivity. Black phosphorus has recently emerged as an exceptional
semiconductor with high carrier mobility and a direct, tunable bandgap. Of
particular importance is the search for exotic electronic states in black
phosphorus, which may amplify the material's potential beyond semiconductor
devices. Here we show that a moderate hydrostatic pressure effectively
suppresses the band gap and induces a Lifshitz transition from semiconductor to
semimetal in black phosphorus; a colossal magnetoresistance is observed in the
semimetallic phase. Quantum oscillations in high magnetic field reveal the
complex Fermi surface topology of the semimetallic black phosphorus. In
particular, a Dirac-like fermion emerges at around 1.2 GPa, which is
continuously tuned by external pressure. The observed semi-metallic behavior
greatly enriches black phosphorus's material property, and sets the stage for
the exploration of novel electronic states in this material. Moreover, these
interesting behaviors make phosphorene a good candidate for the realization of
a new two-dimensional relativistic electron system, other than graphene.
