The X-ray said it all in black and white.
At just over five inches long herself, Polly the tortoise certainly didn't have room for the mysterious egg-shaped growth deep inside her shell.
So vets at Bristol Zoo had little choice but to remove it in an hour-long operation.
They found a stone in her bladder, thought to have been caused by a build-up of calcium, which measured a considerable 1.5in by 1.2in and weighed 19grams.
The stone would have caused the African pancake tortoise a painful death if the X-ray - part of a routine health check - hadn't spotted it.
During the operation, vets cut a hole in the bottom of her shell and removed the stone, before glueing the shell back into place.
Now, encased in a stretchy red bandage, five-year-old Polly is making a slow but steady recovery in the zoo's reptile house.
Sharon Redrobe, head of veterinary services, said: 'We X-rayed the tortoise as part of a standard health check and were amazed when we saw the size of the bladder stone.
'Anaesthetising a tortoise is quite tricky and requires specialist training, but she is likely to have been in some discomfort so we took the decision to remove the stone as soon as possible.
'I've performed bladder stone operations on tortoises before, but never on a pancake tortoise and never with a bladder stone this big.
'Despite initial concerns that we might not be able to get the bladder stone out of the hole we made in the shell, the operation went very well and there were no complications.'
African pancake tortoises are classed as a vulnerable species by the International Union for Conservation of Nature and Natural Resources.
They are most commonly found in Tanzania and Kenya and are named so because of their smooth, flattened pancake-shaped shell.
Several have been hatched successfully in Bristol Zoo's incubators.