prob / O&S - empty tank

What happens when you pump out a tanker without opening the air inlet ...
Atmospheric pressure [about 14lbs per square inch in old money] will crush the tank quite easily.

[American rail tanker]

A similar effect ca be observed if you rinse out a plastic milk or pop bottle with very hot water, screw the top back on and then cool the bottle - it crushes in just like that. The severity of the collapse will depend on how thick the plastic is, how hot the rinse water is, and how well the top seals.
I use this affect to crush plastic bottles prior to putting them in the recycling as they then take up less space.

Not my image : Fair Use & educational purposes