Get the 3 ghosts, then before scanning the remains at the end, head down to the jumping platform part, but instead get to the ledge on the left, from there jump to the cave above, make your way to the other end, at first you won't see it, but there is a ledge bellow, fall down and as you are about to land just jump, then the platforms will appear, take them all the way to the old non functioning portal and fight everything inside.

3 ghosts? What 3 ghosts?