Honey I need a Bigger Safe: 1976 Leopard 1A5 Tank

[Flying10uk]

apparently they are supposed to be filled with concrete

Yer but, there must be ways of removing the set concrete from the said tank. You could "attack it" with a "jack hammer" which may require a fair amount of physical effort or you could "explore" less "labour intensive" methods, such as, immersing the tank in a vat of very strong acid and allowing the concrete to slowly dissolve.