GENEVA (Reuters) - A 100-tonne wheel, the last piece of an ambitious experiment that scientists hope will help unlock the secrets of the universe, was successfully lowered into an underground cavern on Friday. It is the final major element in the ATLAS particle detector, the largest of four detectors being hooked up to the world's most powerful particle accelerator which the European Organisation for Nuclear Research (CERN) hopes to start up around the middle of 2008.