(http://img690.imageshack.us/img690/7632/regolitpit.jpg)
Regolit pits
On Mars there exists a very common and DEADLY! phenomenon colled "Regolit pit".
As you know - Mars is full of dust (seen for example during "Dust storms").
The thing is that the avarage dust particle size differs from that on Earth.
They are MUCH SMALLER (thus - dangerous to mechanical equipement).
When a hole exists on Mars (for instance: a meteorite hole) it will gather all of the regolit dust that sediments there day after day.
Over time it will be filled with that small particle dust.
That's where it starts to be deadly.
Regolit pits are more deadlier versions of a quick-sand.
You will instantly fall inside becouse the dust gathered there behaves more like water then a solid matter.
Someone might drive a rover inside one of those holes and not only destroy it but also make it stuck there permanently.
Also the driver will have one hell of a problem to get outside the rover...
The same goes for a player that doesn't looks where he steps, and falls inside.
Moving inside that pit is very hard, and you do it in complete darkeness. If you were not hurt from falling inside there, you will die of lack of oxygen/power or dehydration.
The only good thing about regolit pits is that they have a different colour then the rest of the ground, so if you are carefoul walking/driving even at night or during sand-storms, you should see them from distance and be just fine.