There are spoilers for Control as well as the trailer for Control Resonant. Read at your own risk if you haven't played the game.
<< Replaying Control, part 16: The Nail (1)The Nail
On the way to the Canyon Rim, Jesse travels through some caves, suddenly falls through the floor, and ends up in the Astral Plane. Specifically, the Astral Collision – the part of the Foundation that's colliding with the Astral Plane. It's mostly Astral Plane with some Bureau equipment and red sand from the Foundation.
Since this is technically part of the Oldest House, there are Hiss here as well.

Traversing the Astral Collision takes quite a bit of time. It never felt like a slog, since the place is beautiful and enemies don't show up too frequently.
There are plenty of collectibles here, including:
- A recording stating that Dr. Ash was the one who created the control points to stabilize the Oldest House
- A notice stating that the Oldest House does not have windows, and that anyone who sees a window needs to close their eyes immediately and go to the nearest safe room
- At least one maneki-neko
- An episode of Threshold Kids that's even more cursed than usual

The Threshold Kids episode is made even more ominous by the ever-present Black Pyramid of the Board in the background.
At the "end" of the Astral Collision, Jesse touches a rectangular pillar with a tree etching in it and is once again taken to the part of the Astral Plane with the sphere. Before she's allowed to do anything, the Board apologizes for being angry with her earlier. They'll let her keep the power Former gave her. That doesn't mean they trust former – they tell her not to listen to it.
This time, the sphere was crystals growing out of it and chunks missing, so Jesse needs to use both powers to repair it. Once that's done, the Nail is a little more repaired.

Now it's time to go to Research Site Gamma. This place is actually located at the entrance to the Foundation, in the caves, but couldn't be reached at the time since Jesse didn't have either new power.
But before we do that, there's something to check out first.
Found Footage
There's a sign pointing to a Chasm. At the bottom of the chasm, past a wall of crystals, is a TV playing a video of Ahti. The TV itself isn't an Altered Item, but the video tape it contains is.

FBC staff filmed Ahti. This created a video tape that, when played, mesmerized all those who watched it. That included the hostile creatures in the Foundation, the Id. Those Id are extremely hard to kill, even with the Service Weapon. We haven't encountered too many of them so far, but this tunnel is full of them.
What Jesse has to do is traverse the tunnel while using the TV to placate the Id. This is easier said than done. There are also Astral Spikes in this tunnel. And given how dark it is in here, you're not going to see all the Id and some will be getting attacks on you. I ended the mission with just a sliver of health.
The NailThere's a sign helpfully pointing toward Research Site Gamma, but I didn't take a screenshot for some reason so you'll have to take my word for it.
Entering the caverns gets Jesse another hotline message from Marshall. In this one, she explains that she blew up the Nail so the Hiss couldn’t corrupt it. Jesse doesn’t like that, but doesn't have much time to think about it because there are Hiss and an Astral Spike in the next room.
Then it's time to head to the Deep Cavern. The cave system leads back outside, with Jesse needing to levitate across a few Firebreak-like natural bridges to get to the Deep Cavern. She can call a Ranger for help in this area. You might want to, since a pretty extensive battle against the Hiss takes place. There are even Hiss snipers present.
When that battle is done, Jesse is now able to access the last of the stone pillars with the tree etching. This takes her to the last of the locks. Once she's repaired the sphere, she goes to leave and…can't? The Astral Plane starts collapsing. Former tells her to leave. Something – maybe Former, maybe the Astral Plane itself – builds a crystal bridge to lead Jesse to a safe area where she can leave the Astral Plane.

Now Jesse is back in the Foundation, and it's time to start the last mission of the DLC.
Or it would be, if I hadn't taken a detour first. Remember those maneki-nekos? It's time to gather all of them and see what they do.
The Maneki-nekosThere are eight of these cats to collect, and I only found a couple on my own. I had to use a guide to find the rest. Some of the puzzles are easy to solve, and some I never would have even known were puzzles at all.

Once you have all the cats, they line up in front of the Firebreak door, beckoning Jesse inside. And what is inside, you might wonder? Well, it's this: more maneki-nekos, and a platform that contains cat ears for Jesse to wear.

Jesse will be wearing the cat ears for the rest of the game. Which is only one more mission because in the next part, we're finishing up the Foundation DLC.