i believe what makes the big ankle bone on the outside is the fibula. So right below that - there is a protrusion (differing in size from person to person) that appears to be from the calcaneus:
Pic - courtesy of here: Ankle Anatomy - Orthogate
So assuming that's correct, the calcaneus is where i'm feelin this unhappiness.
I can see it now - kid falls over on the mountain, grabs his boot, and starts screeming "owww, my calcaneus! my calcaneus!!!"