I've been to a couple areas outside the intended lighthouse, but I didn't make it into the underside specifically. A teammate did, though.

You know that large energy wall in front of the sun barzier? When it pushes you back? The bottom right corner doesn't push you. So if you get up next to the right corner, then jump to the middle it repels you like a speeding bullet. And if you happen to hit that sun brazier properly, it kills you like when a phalanx shield slams you into a wall.