The Underpass Resonant sends Dylan Faden into an Astral Plane incursion that has bled into a tower of floating apartments. Clearing it means opening three sealed doors, each locked behind an Astral Key, before facing The Physicist at the heart of the rift. The route loops back on itself constantly, so knowing where each key sits saves a lot of backtracking across the void.
Quick answer: Fast travel to The Spire in Underpass 3, climb into the light gateway, then gather three Astral Keys (dining room table, bed inside an iron cube, and an inverted-corridor desk) to open all three doors and reach The Physicist boss.
Reach the Astral Plane incursion from The Spire
There is no map inside the incursion, so treat the long central causeway as your anchor. Every key path branches off it, and every door sits along it.
Find all three Astral Keys
Three yellow double doors block the way forward, and each one opens with its own Astral Key. The doors sit in sequence along the central corridor, so you open one, walk through, and immediately find the next locked door with a fresh objective.
| Door | Objective | Key location |
|---|---|---|
| Door 1 | Find the Key and Unlock the Door | Glass display on a dining room table |
| Door 2 | Find the Key and Unlock the Second Door | The bed’s mattress holds it within an iron cube frame |
| Door 3 | Find the Key and Unlock the Third Door | Office desk in an inverted apartment corridor |
Door 1: The dining room key
Cross the fragmented rooms until you hit the first solid yellow double door. It asks for a key, so backtrack across the suspended pillars and domestic platforms to search. Expect an ambush from floating rocky debris clusters when you touch one of the supply crates.
The first Astral Key sits inside a glass showcase on the dining table of an intact dining room module suspended in the void. Grab it, return to the door, and insert it to open the way and collect Physicist’s Log #4.
Door 2: The bedroom key
Through the first door you meet the second locked door. Run down the long causeway and jump onto the disconnected platforms to the right. If you slip into the abyss, Dylan respawns safely on the previous platform, so a missed jump costs nothing.
Push through the lower rooms and a fight with heavy cluster enemies in a narrow chamber. The second Astral Key rests on a mattress inside an iron skeletal cube in the overturned bedroom. Take it back to unlock Door 2, which reveals a corridor ending at Door 3.
Door 3: The inverted corridor key
For the final key, run down the extended causeway toward the opposite end of the void and platform-hop across the vertical apartment fragments. Cluster entities guard the isolated floors and the narrow perimeter ledges along the way.
Climb the rising monolithic steps of the central pillar, then move through the inverted living room where Physicist’s Log #5 sits on a dark ledge. The third Astral Key waits on a desk in the exposed room at the end of that path.
Retrace the route across the void, pass the health station, and leap back onto the central hall. Use the third key on the final double door to enter The Residence.
Logs and the health upgrade worth grabbing
Five Physicist’s Logs (#1 through #5) are scattered through the incursion, alongside two written documents that fill in the story of Stanlov, a physicist whose spatial obsession led to his rejection from Ithaca University. You can read the Letter From Howie and the Rejection Letter in The Residence once Door 3 is open.
- Near the beginning, find Physicist’s Log #2 next to a toppled bookshelf.
- Physicist’s Log #3 — in a side apartment module reached by wall-shifting.
- Physicist’s Log #4 — collected as you open the first door.
- Physicist’s Log #5 — on a dark ledge in the inverted living room.
The most useful pickup is a permanent stat boost. Destroy the retro toaster-like shrine device on the bedroom route to claim Health Upgrade Point 3/3, which raises Dylan’s maximum health once you have collected all three.
Defeat The Physicist boss fight
Inside The Residence, examine the letters if you want the story, then step into the glowing Astral Rift in the living room. An inverted pyramid opens and expels The Physicist, a spherical Resonant built from rotating architecture and multiplied bodies. The objective becomes Kill the Astral Plane Resonant.
Once the entity is dormant, approach the curled radial form and commune with it to calm the Resonant.
Choose Spike or Astral Rebuke, then craft the Amulet
Calming The Physicist opens the Combat Ability Selection screen with two options. Both cost 250 power to use, and you keep only the one you pick.
| Ability | Damage | Falter | Effect |
|---|---|---|---|
| Spike | 317–367 | 9–64 | Summons a spike that attacks or knocks down a target; builds Astralized |
| Astral Rebuke | 459 | 57 | Homing spike erupts beneath enemies and lifts them; builds Astralized |
Spike is the directable, single-target pick that can knock down a chosen enemy, while Astral Rebuke hits harder and throws groups into the air with the Lifted effect. After choosing, you can test the ability on an Astral target dummy in the white space before moving on.
Back in the dark stone chamber, activate the shrine to open the Artifact Formation menu. Here you can craft the Amulet, which increases Critical Chance for all melee attacks by +5%, using Source and collected Echo materials.
The quest is finished once The Physicist is defeated, the Resonant is calmed, and you have made your ability selection. The Underpass Resonant closes out with the new combat ability equipped and the artifact shrine unlocked, so seeing the ability tutorial and the Artifact Formation menu confirms everything registered correctly.
