This small vertical shaft looks like an entrance to the cavern complex blocked only by a slowly spinning fan.
Type: mechanical; Knowledge (engineering, nature or planes) DC 31; Disable Device 24
Trigger location; Reset repair
slow fan (DC 28 Acrobatics check; a failure requires a DC 28 Reflex save or 5d6 slashing damage plus 3d6 electricity damage)
The gravity in this area is lower than what the adventurers are used to; this is just one example of what can be done to both exploit the lower gravity, as well as introduce the PCs to their new environment, before something even more devious is used. Other examples include creating places where the party has to jump chasms, where they can either over-jump the target, or jump higher than they intended, or even taking advantage of the slower falling rate to give enemy spellcasters or archers more time to hit the PCs as they fall. Remember that although some simple actions can become complex, some very difficult actions can be easily accomplished.