A short walk inland from Mnajdra, on the limestone plateau above the cliffs, lies a small cluster of bell-shaped pits cut deep into the rock: the Misqa Tanks. They are usually interpreted as cisterns associated with the temple-building culture of the fourth millennium BC — large enough to hold significant volumes of rainwater, and roofed by a single capstone with a narrow opening.
The tanks are unmarked, unsigned, and easy to miss. Most visitors to Mnajdra walk straight past them. That is part of the appeal: a quiet, slightly mysterious site that adds another half-hour to the temple visit if you know to look. Information at the Heritage Malta visitor centre will point you to them.