OOC: Also note that, in case this wasn't clear before, there is a 20 ft. drop from the end of the shaft (which ends in the middle of the ceiling of the room below) to the floor of that room. There is no way to climb this. You either just drop or bridge the gap some other way (such as with a rope).
The drop itself is not too bad, I'll judge it to be a 1d6 damage drop if you hang from the edge of the shaft. According to the falling rules, if you drop intentionally the first 1d6 are considered non-lethal. If you make a DC 15 Jump of Tumble check, you can ignore this 1d6 entirely (note that armor-check penalties apply). There is, however, the small problem of getting up again once you've dropped down ...