  We review recent advances in diamond anvil cell development, focusing on the
details of magnetic susceptibility techniques. We have developed and
implemented highly sensitive technique that allowed observations of
superconductivity using magnetic susceptibility at megabar pressures, including
the observation of a Tc in sulfur to above 230 GPa. The advancement of the
technique should enable the most demanding diamond anvil cell applications. The
Tc(P) data for simple s-p metals (Se,S, MgB_2) are presented and analyzed. The
possibility for a phonon-assisted electronic topological transition (ETT) in
MgB_2 at high pressure is discussed.
