For the first time multiwall carbon nanocells (circle divide < 100 nm) and multiwall carbon nanotubes have been artificially grown in hydrothermal fluids from amorphous carbon, at temperatures below 800C, in the absence of metal catalysts. Carbon nanocells were the result of bubble growth of graphitic multiwalls at 600 degreesC. Pristine multiwall nanotubes grow at higher temperatures. At 800 degreesC, the condensed solids consist of defect-free sp(2)-bonded multiwall nanocrystals. Hot hydrothermal fluids seem essential far the growth of curled patches of sp(2)-bouded carbon at low temperatures. Hydrothermal synthesis of finite sp(2)-bonded nanotubes and carbon nanocrystals is possible in moderate conditions and easy to reproduce.