Double-walled carbon nanotubes were pressurized to similar to90 GPa by using a diamond anvil cell (DAC) at room temperature. High-resolution TEM and Raman spectroscopy on the recovered sample revealed that the carbon nanotubes collapse and result in the formation of a highly compressed carbon form. This carbon form has a c-layer interdistance of 0.21 nm, which is remarkably shorter than that of graphite (0.34 nm). The calculated density is 3.65 g/cm(3), slightly greater than that of diamond (3.52 g/cm(3)). It is suggested that this dense carbon form is a potential superhard material and is likely the significant precursor of the phase transformation of graphite to diamond.