  A novel allotrope of carbon with $P2/m$ symmetry was identified during an
\emph{ab-initio} minima-hopping structural search which we call $M10$-carbon.
This structure is predicted to be more stable than graphite at pressures above
14.4 GPa and consists purely of $sp^3$ bonds. It has a high bulk modulus and is
almost as hard as diamond. A comparison of the simulated X-ray diffraction
pattern shows a good agreement with experimental results from cold compressed
graphite.
