The story
It can be visualized as the problem of taking a square arrangement of cannonballs on the ground and building a square pyramid out of them.
Etymology from Wiktionary, CC BY-SA 4.0.
cannonball problem
Today it means: The challenge of proving that the only solution of the Diophantine equation :∑ₙ₌₁ᴺn²=M²; with N > 1 is when N = 24 and M = 70. Full definition →