Complex sentence: A heavily fortified U-boat submarine base was built by Organisation Todt shortly after occupation, with its 9 m (30-ft) thick concrete ceiling was capable of withstanding almost any bomb in use at the time.
Split into simpler sentences: A heavily fortified U-boat submarine base was built by Organisation Todt shortly after occupation. Its 9 m (30-ft) thick concrete ceiling was capable of withstanding almost any bomb in use at the time.