Nothing out of the ordinary, just native Ohioans

Do you need to know?

It could be interesting to have it just be a canonical "black hole, ie nobody knows what's there - it started with people, animals and plants disappearing and then jumped to hills, buildings and even weather systems until there was nothing there at all as far as the outside world is aware; nobody who goes there ever comes back, satellite photos come back corrupted, and people are scared to even mention it

What are the rules of the world?

Ohio is a perfectly nice place filled with perfectly nice people. Exceptionally, creepily, contagiously nice.

It's Ohio in an alternate timeline. They don't want you to know what you're missing out on. But some evil lurks beneath the surface, of course.

Almost at the top of the rope. You won't have long before a patrol spots you, but long enough to snap a picture and get out. You swing your legs over and look around.

The top of the wall exends into the morning haze in both directions. The view to the other side is blocked by a jumble of small huts, chimneys, floodlights; each one emblazoned with the logo of the Ohio Containment Corp - the block capitals OHIO enclosed in a möebus band. You pick your way through until you see what you came here for.

Disappointment. It's just the same scrubland, dusty tracks and fields that you drove through to get here. Maybe there's some settlements way off in the distance? It's hard to see. Oh well. What did you expect? The whole secret laid bare on your very first view? You snap a picture, and step forward for a better look.

What was that? You heard something! Is there someone else here? You dart back and crouch, out of sight you hope, and try to listen. If there was a guard they would surely have raised the alarm. You creep forward, quietly as you can, then .. shit! You see its shadow. Human shaped, crouched, moving slowly, stalking you.

You've gotta go. Where's the rope? You must have got turned around in the confusion but you find it. Swing over and back down. Hand over hand as fast as you dare. Your buddy Flip is still at the bottom where he parked the car.
"Whoa, slow down. You'll break your neck"
"Flip, we gotta go"
"Did you get the photo?"
"Yeah, but -"
He grins and raises his palm. You fumble the hi-five.
"Flip, someone saw me. We gotta go"
He looks up at the wall, sceptical, but nods. "Hey, are you driving?" He motions you towards the other door. You stumble around and climb in. What is wrong? Something is, you know it.
"Wow, you're really flustered about something. You'll feel better with some breakfast." He hands you a candy bar. You take a bite. It tastes .. not bad, but different. It's some knock-off Snickers bar with its label in a foreign alphabet. Flip starts the car and pulls out left, past the Containment Corp sign. You notice a misprint on the logo - it says OIHO.
"Flip, I don't think this is the way back"
"Sure it is - it's the same way we came here. You're really in a state. What happened to you up there?"

You don't know. You try to think. You try to reflect.

It could be an area for nuclear waste. Long term nuclear waste signage is an interesting concept. How can you make sure an area is understood as dangerous for 10,000 years?

Here is a Wikipedia article on it but ideas include fields of thorns like this image

but you could add other elements like menacing rock blocks.

If you want you could have mutated wild life that inhabits the zone.

If I recall correctly some places employ people to shoot birds and what-not that enter so a military precense may make sense for several reasons.

Bonus for those interested here's a BBC article on the concept

https://www.bbc.com/future/article/20200731-how-to-build-a-nuclear-warning-for-10000-years-time

How "settled" are you to it being based in Ohio? Sorry, but all the memes a few months ago just ruined my ability to take "The Ohio Exclusion Zone" seriously. Maybe consider basing it near Lower Appalachia? Like West Virginia through western North Carolina? There are plenty of Appalachian folklore monsters you could use for the area. Wendigos are one.

Hell, you could even have some kind of nuclear leak from Oak Ridge, TN, be the cause.

Have you ever read Roadside Picnic, the book the STALKER stuff is loosely based on? I highly recommend it and I love that they really have no idea what The Zone is. The best suggestion was from a scientist saying that we were like ants goggling at the trash and crumbs left behind after some beings beyond our ability to comprehend had a roadside picnic during a trip to our planet and left it all behind. We never witnessed the beings themselves, but their refuse is like gold and/or poison to us and we can barely comprehend any of it.

There were many immediate dangers within The Zone, but I like to think the main reason it was closed off was because some anomalies acted contagious to materials, such that metals could contract the effects and spread them to other metals. Imagine that getting out.

Just wind turbines as far as the eye can see.

The noise and "radiation" is contained by the wall.

It's Ohio. It's filled with stupid. (Source: grew up there.)

Have you seen the show Threshold?

A crystalline growth which can make people go mental or take over them to become worker drones.

Alternatively, sticking with the crystal theme. Tiberum.

play into the weird horror.

Ohio is a wasteland of eldritch horror, the likes of which drives those exposed to a state of insanity.

those guarding the border are administered anterograde amnesia drugs before every patrol, preventing formation of long term memories and preventing them from going mad.