Rrrg, I've only got two Riddler Challenges left but I don't remember how to get to them!
They're in the caves, more specifically the pump control rooms, in two rooms that intersect with the main hall where you fight some goons and a Titan. When I tried to get there the way I went in, there's an electric barrier blocking the entrance. I don't remember how I got out of there, can anyone help?
EDIT: It's the Pressure Control Junction in the caves. There's an exit point on the map that says "Arkham Island, West" but I don't remember where that outlet is!
EDIT #2: Nevermind, I'm stupid. Never noticed the power cables running back past the doorway... found the security control to override the electric barriers.