It's not an underground reservoir, it's an artesian well!
Well, you are correct of sorts, but in 1984 when I worked there, Higsons were pumping up many thousands of litres water from the underground lake which they called Liquor to use in the brewing process, this gave Higsons ales a unique taste as the Liquor used contained dissolved minerals from the local Limestone.
Also many other factories were using well water for their own processes which caused the water table to drop to a level where an underground cavern appeared under the brewery, I was told at the time it was so big the employees could take a boat ride on it, many inscriptions around the walls could be noted from as early as the 1800s.
Sadly as many factories have now pulled out of Liverpool and stopped using this natural resource the water table has risen and flooded the cavern. Ufortunately I never ventured further than the third ladder at the time as I was only 18 and got "the fear".
The picture of the well is an older well that predated the one with the pumping equiptment over it.