I'm basing this report on the notes I took while I was searching. I'm afraid that I didn't take as detailed notes as I should have during the search. Reconstructing things from memory, I had been searching the woods for nearly two hours when I encountered the anomalous location (in the grid G38 in my notes). There were no easily apparent visual signs, and my first notice was when my compass needle jerked. I thought that I had accidentally tipped it too far, so I stopped and held it steady until it returned to magnetic north. I now suspect that something about the gates may disturb magnetic fields slightly, but more testing is required.
While I was stopped, I happened to look around. There was a large fallen tree nearby. I hadn't been paying attention to it because it was the same tree that was also blocking F38 and I had to climb over earlier that day. This time my path detoured around it's base, where it's torn roots were hanging in the air. But a little ways up the trunk was a carving that had apparently been there while the tree was still alive, since the bark had healed over it. Three straight lines and one curved.
I wouldn't have noticed it if I hadn't been looking, because it was facing toward the ground and half-hidden in the shadow of the trunk. Crouching down, I put one hand out to steady myself and found myself slipping. What I had mistaken for a shadow under the tree was actually a depression, a depression that continued under the trunk. A dark tunnel, in other words, very similar to the one that Ariel had described to me.
I used my cell phone to get the GPS coordinates and to note down the time and I took out my flashlight. The LED-bright beam confirmed that the tunnel extended at least ten feet, sloping downwards. I couldn't see any further than that because of the angle, but I could see enough to tell that the far end of the tunnel had a natural stone floor.
There was just one problem: I would have to crawl. The tunnel was way too low for me to do anything else if I wanted to see the other side. It was a good thing that I wasn't claustrophobic. I had been prepared for a caving expedition, though, so I clipped my second flashlight to my belt, sent a text to Ariel, and left a note in a plastic bag taped to a nearby sapling. It'd be better if I had a partner as a backup, but I didn't really want to involve anyone else until I was sure that this wasn't completely crazy.