I'm not sure this is really a tunnel. It should be called "the extra-large pipes" or perhaps "the sewer" since either description would fit better than "the tunnel." To me, a tunnel is large enough to drive a car through, is dark but devoid of light and, most importantly, has breathing room.
The Mud Run tunnels have little room to breath in or otherwise. Unless you are two feet tall, you have to crawl through, and from what I remember last year knees can get scraped.
These "tunnels" are very close to the end, so by the time you get there, you are almost done which means you are pretty much out of gas. I remember dragging through this tunnel, my knees slightly complaining of pain but the rest of me too tired to care.
Luckily, these "tunnels" appear only once.