This thread is for those who want to enter opposite version exclusive UB worlds, as they exist in the game files but are inaccessible.

You require PKHeX to alter your map location.

Disclaimer: Make a backup save before doing any of this.

I suggest entering a random wormhole (white pref) to prevent crashing or game errors.

Pheromosa
Map: 251
X: 2875.50100
Z: 11077.22000

Buzzwole
Map: 253
X: 4247.17000
Z: 9168.49000

Celesteela
Map: 250
X: 6081.94400
Z: 5828.67600

Kartana
Map: 252
X: 2195.99900
Z: 7419.94500

If anyone has working Ultra Ruins coordinates that don't crash the game, we'd like to have it.