More info from Mary Caldwell:

This was quite an undertaking as cement passed from person to
person by bucket up the side of the hill. It took time, but was eventually
completed and included a huge cistern at the base to hold the water pouring down
the hillside on the catchment area. So with a heavy down pour, it was like
Niagara Falls.

Fresh water was also needed to construction and as the homes (designed by Mary) were constructed the cisterns were built first to hold rainwater from the frequent squalls over the island.