The Underpass Resonant is the regional boss hiding at the top of The Spire, inside the Astral Plane. The fight itself is not the hard part. Getting to it is, because the boss room sits behind a chain of locked doors, and none of the Astral Keys that open them show up on your map.
Quick answer: Climb The Spire into the Astral Plane, then open three locked doors in a row. Each one needs its own Astral Key, found by branching off from the door and looping back.
What you need before starting the Underpass Resonant quest
This is a late-ish Resonant, and the Astral Plane section assumes you already have the movement tools that Jesse’s Faults hand out. If a platform looks unreachable, the problem is usually a missing ability rather than a missing path.
- Shift, unlocked by completing The Incursion Fault, for changing which surface counts as the floor.
- Gravity Anomaly, used constantly here for running up walls and flipping upside down.
- The Underpass Resonant objective active in your Quest Log, which reads “Kill the Underpass Resonant”.
- A Blunt damage weapon, which makes the armored mini-boss on the way up far less painful.
If the combat between keys is what is stopping you rather than the routing, Assist Mode has immortality and one-hit kill toggles. The platforming still has to be done manually.
Reaching The Spire and entering the Astral Plane
Fast travel to the Spire point if you already have it. If you do not, use the Underpass Fast Travel Door and follow the quest marker across the zone’s shifting platforms and rotated roads.
The approach runs over red-carpet platforms and broken concrete ledges, with plenty of vertical jumps between them. Take your time reading each gap before committing, since a missed landing drops you into the void and costs you a respawn.
The truck strung with party lights in the lower room is a useful landmark if you are second-guessing your position, and the hallway past it leads back toward the marker.
All Astral Key locations and the locked door chain
The doors work as a stack. Open one and there is another locked door directly behind it, and each new door sends you back out into the void for a different key. Every key sits inside a glass box at the end of its branch, and picking one up shows a +1 Astral Key pickup message.
| Key | Where to break off | Landmark at the key |
|---|---|---|
| First | Drop to the lower platforms beside the door | Trail of desks and office gear |
| Second | Turn around, run to the far end of the path | Upside-down platform past the cubes |
| Third | Turn around, drop onto the platform on the left | Four chests next to the glass box |
First Astral Key
Interact with the double doors first so the game registers that they are locked. From there, drop down to the lower floating platforms and follow the path until you hit a chest. Opening it spawns three geometric orbs that chase you and ram into you. They hit reasonably hard but have no other attack, so keep moving and pick them off one at a time.
Proceed to the next chest at ground level. Opening it activates a floor-wide spike sweep, so retreat and wait for the hazard to pass. Next, use Gravity Anomaly on the climbable wall and follow the desks and office equipment all the way to the key, which is inside a glass box. You will also pass an audio recording and a file placed on the desks.
Carry it back to the door, open it, and the second locked door is immediately behind it.
Second Astral Key
With your back toward the door, sprint along the path and leap over the gap at its end. As you approach the checkpoint marker, a purple skull appears and three more geometric enemies spawn. Defeat them, cross over to the right, and turn left; approach the following marker slowly because it activates spikes in the ground.
Once the spikes retract, run up the wall, jump the gap, and head left. Climb the cubes on the right, cross the next gap, and use Gravity Anomaly at the edge to walk along the walls until you are fully upside down. Turn 180 degrees and the platform holding the second glass box is in view. Grab it, open door two, and door three appears behind it.
Third Astral Key
Turn away from the door again and drop onto the platform to your left. Launch across the gap and follow the path to a chest, which will vanish as you get close and reappear on the next platform. This one is a chase. Keep following it until it lands on a wall to your left, jump across, and switch planes so you are running along that wall.
Remain with the chest positioned along the walls until interaction becomes available. Opening it calls in a group of enemies. After defeating them, go to the platform’s back-right corner to rejoin the route. The path ends at the last glass box, and four nearby chests contain upgrade materials.
A Health Upgrade Point (1/3) sits on this stretch, tucked in a yellow cat dispenser on a raised ledge near a stove and refrigerator. Collect three of these across the game to raise your maximum health, so it is worth the detour while you are already here.
Bring the third key back and the final door opens. Beyond it, a corridor leads into a room with a glowing white rectangular portal. Stepping through starts the boss fight.
How to beat The Physicist
The Physicist is the Underpass Resonant, and its health bar is labeled with that name at the top of the screen. It punishes greed more than anything else, so think in short attack windows.
Phase one
Run the moment the fight starts. It chases you and detonates a large area-of-effect explosion when it gets close, repeating that several times before rising into the air. From up there it fires three waves of four projectiles, all of which you can outrun by sprinting in a wide circle around the arena.
When it drops back down it sweeps the ground around its base, so jump and attack from the air to stay above it. Otherwise, wait for an explosion to go off, dash in, land about three hits, and get out. Keep that rhythm and it eventually tears up the arena and retreats along a newly opened path.
Phase two
You land in a second circular arena and the boss splits into four smaller copies of itself. Treat them like the orbs from the key routes and take them one at a time, because a couple of hits in quick succession can stunlock you.
After they die The Physicist reforms with its old moveset plus two additions. It flies off and sends waves of spikes across the ground, which you dodge left or right while closing the distance for free hits. It also pulses without exploding, launching spikes up from under your feet instead. Hit it during that wind-up, then dash sideways the moment the pulse starts.
Phase three
At roughly 50 percent health it flees once more to a final platform. Here it summons two orb minions and stays in the fight with them, which is the most dangerous stretch. Kill the minions first, taking your openings between the boss’s attacks, and prioritize staying alive over damage.
With the minions gone, nothing new gets added. Go back to punishing the explosion, sweep, and spike attacks until its health runs out.
Rewards and confirming the quest is complete
Killing The Physicist gives you a choice between two Combat Abilities, plus the Untapped Golden Earrings artifact recipe.
- Astral Rebuke — tap R1 to send an underground spike traveling to your target and hitting it from below, leaving the target airborne.
- Spike — the alternative Combat Ability offered at the same prompt.
Claim the ability from the Astral pillar; a confirmation message and its control prompt will then appear. Hold Up on the D-pad to exit the Gap. You return to the Underpass, and on PlayStation the Astral Reconfiguration trophy appears, confirming most clearly that the entire chain registered.
If you are still working through the Underpass, keep an eye on the quest name in the upper-left corner before you move anywhere. The region can hold several objectives at once, including a Jesse Fault and the research team route, and they all send you across the same tangled geometry without being the same task.
