That was surprisingly effective. I kept waiting for something to happen that would end things but it never did. The flashlight dying seemed like it should indicate an ending but no. Then I thought maybe I'll hit a dead end or fall through a hole or the tunnel will get too tight to crawl through but... nothing. And that somehow made things far more unsettling. I don't know if there is an end or if it does just keep going forever but I'm very impressed at how much atmosphere you created with a such a simple premise.