You have to feel sorry for this poor kid, BUT I imagine this prompted him to ask "why" is that one deeper than the others! It also may have scarred him for life and he'll never explore another mud puddle, sandbox, or swiftly moving river again! But seriously, that's a deep hole with no real evidence explaining why... I've seen potholes like that before, but not puddles! I'm guessing man made (like when I tried to dig to China), but maybe you have some other ideas!