In Control Resonant, the Unknown is a hidden zone that only unlocks once you reach certain milestones in the story. Accessing this zone is far from simple, too, of course, as you'll also need to meet some prerequisites and take several steps before you can even access and explore the area.
If you're wondering how to unlock the Unknown zone in Control Resonant and explore it freely, then this guide will walk you through the entire process, starting with the Hedron takeover.
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How to Unlock the Unknown Zone in Control Resonant

In Control Resonant, in order to unlock the Unknown Zone, you will first need to find and complete the Oldest House Fault, which is also the final Jesse Fault objective. After completing this sequence, you will be able to begin the quest known as "The Hedron Link".
First, find and speak to Pope to learn more about the Hedron. After this discussion, continue through the Main Test Chamber, and use your abilities to shift gravity to one of the walls, changing your perspective.
Clear out the enemies along the way and continue progressing through each door and staircase, heading up through the levels of the building. Eventually, you will come to a section of fragmented building pieces. You will need to float towards them and use Reach to propel yourself forward to each platform, crossing over to the next section safely.

After making your way up to the rooftop through the elevator, you will come to a bright, yellow and orange distorted version of the facility - this is the entry point for the new Unknown Zone.However, you will need to complete a few more steps before you can actually reach the physical parts of this zone and explore it properly for your story progression.
How to Reach & Explore the Unknown Zone

After emerging from the elevator, you will be able to see floating balloons set by Pope. Make your way over to them and hit the balloons to drop them and open the sensors inside the crate.

You now need to place all sensors in the appropriate locations. Much like earlier quests, this is indicated by the beeping frequency of the sensor itself.
For the first sensor placement, use your abilities to climb to the top of the structure, and then use the beeps from the sensor to help you find the red circle seen in the screenshot below. This red circle is the exact spot in which you'll need to place the first sensor.

Now simply grab the same sensor and use the same method to place it, this time over the other side. After placing both sensors, a strange energy disc will appear in the sky.
Use your reach ability to travel towards it and land on top and now press the jump button to launch yourself forward. This will send you flying through the air towards the physical Unknown zone, letting gravity carry your freefall towards the city terrain.

As you grow closer, use your reach and shift abilities to ground yourself to solid platforms and flip the world around as needed for your exploration. You'll now be able to explore the Unknown freely and continue on with the objectives Pope has set up for you in anticipation of taking down the Hedron.