The Frognest Tunnel

The Frognest Tunnel is a hand-carved train tunnel deep in the Warrior Coal Fields north of Birmingham. Built in the 1880s, the tunnel was unfinished, bare rock. The tunnel had an inherent design flaw. It was lower in the middle than at the ends. This resulted in the tunnel regularly filling with a couple of feet of water. The tunnel was eventually abandoned, replaced by an above ground cut nearby. Since its abandonment, the tunnel has fallen into deep decay. The mortar and steel beams that once reinforced the ceiling of the tunnel have given way to rust, water drops from nearly every inch of the ceiling, and huge chunks of rock that once were securely embedded into the ceiling and walls now lay in a few feet of water at the tunnels bottom. How many years more will this tunnel bear the test of time, before it totally gives way and collapses in upon itself?