For my birthday ride, we headed out early on this lovely morning. We rode from Jessup through the industrial park south to Marshwood Road (passing by the gate to Rock Slope); we first checked out the area near the lower powerline cut. No tunnel portal. Following a rocky, muddy path east (my first real off-road riding this season!) took us to the higher powerline cut, which we then rode back out to Marshwood Road. Not 30 feet from the road, just west of the pole line, we saw a solid concrete slab that looked like the top of a fancy culvert. Could this be it?
We fought our way through the thorns and brush and discovered that it was. Cold air issues from within the tunnel, in a very similar manner to Pocono Cold Air Cave. The tunnel floor is visible some ten feet below the surface. Railroad ties and other rotting mine artifacts are visible down below despite the near-darkness. As we had noted from photographs, "Tunnel" is spelled with an extra L on the face of the portal.
On an unrelated note, we finished off the day’s adventures by riding back to Rich’s place for some well-deserved birthday hotdogs, snacks and dessert before work!