C'mon Guys,
Its a pool to test for water tightness, after a ordnance test.
I'll explain it slowly…
Tanks, armored vehicles MUST be completely sealed, to prevent the intrusion of hot pressurized gas from an explosion, above, or close to it.
If not completely sealed, the gas will enter the vehicle, through the tiniest gap, and this will result in the complete destruction, of the vehicle and crew.
The Britts developed Bill2 to exploit this weakness, found particularly in Russian tanks.
That's why you see Russian tanks exploding so well. Not completely sealed, you see?