Banana
Very, very seldomly incorrect
Attenuation Tank. Which is to "cache" the water run-off and feed it into the local drainage system over time so as to not overwhelm it.There's a very large storage tank built under the pitch in the South and West Stand corner from memory. The drainage system must be designed to store a huge volume of water in the tank which must explain the pitch drainage being so good.
It was interesting that from some cameras that some parts of the pitch looked poor, but then from another camera the same area looked perfectly ok.
As has been said up-thread, ours is one of the better pitches if you take a look at other games. All a product of last summer's lockdown it would appear.