First go to Forge World and go to the first island.

Next find flat ground, the best place is in front of the mountain beside the water.

Take a platform big enough to hold a Wraith, go as hogh as you can and put the platform there. Set a Wraith on top.

Lastly delete the platform (don't touch the Wraith), get under it and shoot it.

Then the Wraith will come down and splat - you're under the island!