I found these 1-block-wide tunnels that went straight down while exploring. I didn't get to the bottom of them, as you can see by my short tunnel beside the one on the left.
Since I couldn't get to them, I decided to pull up Terramap and take a quick look, and found it. However, there's another one right by it, so now I know it isn't just some weird occurrence. I went to the gamepedia, and I haven't found anything on it. What is it?