According to Wikipedia:

Jones and three others had left their party in search of “The Birth Canal”, a tight but navigable passageway with a turnaround at the end. Jones entered an unmapped passageway which he wrongly believed to be the Canal and found himself at a dead end, with nowhere to go besides a narrow vertical fissure. Believing this to be the turnaround, he entered head-first and became wedged upside-down.