The Phon'Qi Caverns is accessible from the mysterious shack in Foothill Falls, and is part of the "Investigate the Abandoned Shack" rumour that you can get from a Prospector in Rambler's Reach. You'll want to tackle the Phon'Qi Caverns with all of your abilities, so come here once you've finished the main campaign.

It's a good old fashioned dungeon, this, and it comes with an excellent reward at the end. In the final room, there's a piece of machinery to interact with. Do so, and you'll unlock this trophy, along with a Map Upgrade that gives you the ability to see any unfound Chests, Stim Upgrades, and BD-1 Upgrades. Very nice.