concrete fields
Each of the 59 pill-shaped columns weighs about 500 tonnes. In addition to their obvious function of supporting the roof and the 22 metres of ground above, they act as a counter weight against the buoyancy of the cistern in the surrounding groundwater.
It's difficult to appreciate in photos just how huge they are in real life, but to give a sense of scale each column is seven metres long and two metres wide, and an adult standing next to one would be about the same height as the buttressed base.