I made it. Yeah, I did. It cost me everything, but I got through that door. And what I found on the other side was not worth the risk.
Let me rewind a bit, for anyone who finds this and has no idea what happened. If you don't know what the Foundation is, then I don't know how you got this note, and you should drop it right now. I will assume that you do know about the SCP Foundation, and so I'll skip all the warning crap. I don't have any fancy memetics anyway.
I was sent to test on SCP-1346. That's the corridor that freaks everyone out. Anyway, no one so far has made it past 100 meters without running. Except me. I was chosen because I'm different somehow. Something about 3031, being immune, or something. Basically, I have much lower fear reactions than anyone else. I was the perfect test subject. I was shoved in alone with a cable attached to my back, and the door was closed behind me. It was pitch black, so I lit a flare. I was told if I didn't go down the creepy hallway, I would be "terminated". Not likely. If this immunity was so rare, then they wouldn't just kill me. The radio in my ear squawked something about moving forwards. I began walking, holding the flare high to light up the tunnel. There wasn't much to notice. Just a tunnel. About forty meters in I felt like something was off. I looked around, but couldn't see anything. At somewhere like eighty meters, I was sweating. Yeah, things weren't right. I wasn't downright terrified, but I was scared, no question. I commented on this to the radio, and kept moving. I passed a chalk line marking 100 meters. Must have been put there by a rover. The doorway looked closer now, like it was only fifty meters away at most. I figured if I'd made it this far, I could make it a little further. At around about 130 meters, I was freaking out. I felt like I was being watched. I begged to be let back out, but I was denied. So I kept walking. Wish I hadn't now.
I finally made it to within five meters of the door. It was right there, but I was rooted to the spot. I was so scared. I'd never been this terrified of anything. Seriously, I was usually pretty calm, even when I thought I was gonna die. But this was something else. I was so close, yet so far. I nearly collapsed there, and that would have been the end for me, but the suits back at the entrance must have rigged me with a shock collar or something. A huge jolt ran through me, and for a second I forgot my fear. I stumbled forwards, and touched the door. Suddenly, it was like a gate had closed, and I felt fine. I even felt stupid for being afraid of a tunnel. I laughed in relief. The radio crackled in my ear, and I turned back to the door. There was a deep thumping coming from behind it, like the footsteps of a giant. I reached up to the valve, opening it enough for the door to swing open. The footsteps suddenly ceased, and I opened it up. The radio, which I could hear the suits through, suddenly went quiet.
I know whoever's reading this will want to know what was beyond that door. I won't tell you. All I will tell you is it's out now. SCP-1346 is a prison, and I'm it's newest inmate. It must always have at least one thing inside it. So the thing that was in there before got out, and I got put in. Suffice to say, I can't talk about the thing that got out. If I did, this paper would be gone in a few minutes of me sliding it under that huge door. All I can say is, good luck containing it. I mean that.

Note found at far-end of SCP-1346 by electromagnetically shielded drone approximately 1 week after incident SCP-1346-[DATA EXPUNGED].
Research is ongoing into whether SCP-1346 could be used for containment purposes.