I woke the next morning to a steady shaking by the hands of my lieutenant.  The pipe had opened up during the night.  I asked him how, and he said he didn't know.  There were no eyewitnesses among the crew.  The lid had merely slid open, though no hinges or attachments seemed present.

Taking a lamp and shining it down the tube, there was nothing to be found,  it apparently went very deep.  I dropped a coin down it and i heard a clink about 15 seconds later, then a sound that seemed like it slid down.  I ordered the crew to lower a rope and asked for volunteers to go down.  One of the men in my group volunteered, his name was Darius, he was a young guy only with us a few months.  This was his first expedition with us past the Medosa bridge.  He attached his light to his belt and began descending down into the blackness.  after a few minutes he yelled up

"the pipe turns here, should i keep going?"

"is there room to walk?" i replied

"yes"

"good, tie the rope to your belt in case it turns downwards again."

"alright sir" he hollered back up