The branching tunnel slopes upward for about five feet; it's a tight fit, but there's enough room to move one's arms and even bend the knees a bit. The Claw marks have created divots at regular intervals, like ruts on a well worn path, which makes the going even easier.
At the crest of the slope, it abruptly drops straight down again, although Max's enhanced eyes can see that it curves away again after about a fifteen foot drop.
Most likely, it's a p-trap, to keep the well from filling with rainwater and flooding the tunnel. The bad news is that, once Max goes down, coming back up will be damn hard.