This sounds a little far-fetched to me, but the following appeared in Lassen Advocate on May 19, 1922 with the latest update on the construction of Bly Tunnel.
”An interesting circumstance in connection with the tunnel work is the fact that mules are used therein to haul cars used to convey the excavated materials to the incline. These mules were taken down the incline last fall and have remained underground continuously ever since, except a short time ago one of them became slightly indisposed and was brought to the surface for treatment. Upon reaching the surface and encountering the strong light of day, the mule apparently became panic stricken and cavorted around in an unusual manner for some time and finally backed up to the mouth of the incline and descended downward to the tunnel level, holding on to the slats or steps with his front feet. He is now back on the job and will probably stay until the tunnel is completed.”