Come to this place in the Valley of Ashes, and after feeding the crystal giant slime, follow the water column to the back of the waterfall.
The way to the portal is just behind the waterfall (it is recommended that the aircraft be upgraded to level 2)
Detailed location
If the portal has been opened, you can jump directly to the bottom, which is not the sea.
After walking for a while you will find the portal. After placing the crystal, jump directly into the open door.
After this you still need to walk a short distance and fly over the sea in between
Be careful not to fall into the sea, you will die
After crossing the sea, you can see the portal leading to Powder Cliffs.