I found out from Vyse. He's been here about two years, said he was around when the tunnels were first discovered. I wanted to take Toph down there, but when I found out she wouldn't be able to bend down there, I was going to forget about it. Then this morning... it hit me!

I'm not sure how it works, but there were conditions back home where bending powers could be lost. My guess is that there's something in there that is able to find the source of a person's power and... somehow block it. Like chi blocking, but automatic.