As I understand it, there's the cave where Barathus and the others are, and then there's the tunnel we're in, and then there's the giant. Past the giant is the way we originally came when SkySting dropped us off, and assorted other tunnels we didn't go down (including the one the Thri-Kreen ran away through). Past Barathus is where we need to go (up the wall on the precarious poles).