In this pic you can see the access hatches (3 on each side but only 2 on each side visible) giving a view into the six higher level tanks. With six lower level tanks and their 6 access hatches visible in photos of the exterior of the complex.
These mills must have consumed vast quantities of water and energy.
There is much to learn about the operations. Most histories have been written by women authors and are told from a women's perspective and fair enough - it was hard dirty and smelly work and with compulsuion throw in, something the record!