Survey studies of pressure-temperature-induced transformations of single-wall carbon nanotubes (SWNT) at 1.5, 8.0, and 9.5 GPa and temperatures ranging from 200 to 1500 degreesC in a piston-cylinder or "Toroid" type high pressure devices, have been carried out. It was found that the combined effects of high pressures and high temperatures produce irreversible changes in the SWNT structure, in contrast with the reversible effect of,high pressure alone, previously studied by several research groups. The Raman, electron microscopy, and X-ray diffraction data obtained in the present work provide evidence for covalent interlinking of SWNT by sp(3) C-C bonds which escalates with the increasing treatment temperature and pressure. The formation of new carbon structures such as nano- and, microcrystalline diamond-like (cubic and hexagonal) and nanographite phases under higher pressures (8.0 and 9.5 GPa) has been observed.