How to find secret level in DOOM Episode 3: Inferno

In level 6 Mt.Erebus:

It helps to first be Invulnerable. Enter the building near the
center of the map, roughly at the North end of the island, use
the teleporter in the middle of the West wall to go to the
platform in the lake. Go down the ledge to the East, turn and
face the wall so that your back is directly facing the blue
box-like area below. Fire a rocket directly into the wall, the
kickback will propel you right into that area. Tap the skull
and you're there!