Complex sentence: The precise cause of the accident is not known, but as no emergency measures appear to have been carried out it is thought that the submarine dived suddenly and rapidly past her maximum depth limit and suffered a catastrophic hull rupture.
Split into simpler sentences: The precise cause of the accident is not known, but as no emergency measures appear to have been carried out. It appears that the submarine dived suddenly and rapidly past her maximum depth limit and suffered a catastrophic hull rupture.