To find the Fortbyte number 67 of the decryption challenge, you have to go to the center of the map. As the title of the challenge says, head to the "Southern Most" platform that is actually much closer to the center of the map than to its southern end.
The platform in question is located in the plain between Fatal Fields and Salty Springs.
Don't forget to equip yourself with the Retaliator glider (level 79 of the battle pass, paid version). If you don't have the right glider, the rings will not appear as you approach the platform, .
