  Raman spectroscopy has been the most extensively employed method to study
carbon nanotubes at high pressures. This review covers reversible
pressure-induced changes of the lattice dynamics and structure of single- and
multi-wall carbon nanotubes as well as irreversible transformations induced by
high pressures. The interplay of covalent and van-der-Waals bonding in
single-wall nanotube bundles and a structural distortion near 2 GPa are
discussed in detail. Attempts of transforming carbon nanotubes into diamond and
other "superhard" phases are reviewed critically.
