Complex sentence: Though inhabited in prehistoric times, the lower cave was not rediscovered until 1836 by Reverend William Thomson; it can only be visited by boat since it channels an underground river that provides fresh drinking water to more than a million Lebanese.
Split into simpler sentences: Though inhabited in prehistoric times, the lower cave was not rediscovered until 1836 by Reverend William Thomson. The lower cave can only be visited by boat since it channels an underground river that provides fresh drinking water to more than a million Lebanese.