Choking from noxious fumes.
Pockets of explosive gas detonated by flint patches
Loud position revealing pops and crackles from minor sub ground explosions
Sprays of lava, rock and geysers
Plumes of vision obscuring ash
Random pockets of high temperature
Natural pit traps into lava or acid pools
Corrosive sulfur deposits that erode simple gear (clothing, packs, armor straps)
Erupting steam pockets that hurl people into the air
Stuff like that?
Are you looking for examples specific to this scenario? The basin could collapse within itself and bury them alive.
Why do you actually want to kill players from a single bad roll? Seems like an awful idea to me.
Coming up with meaningful hazards is reasonable, but if the hazard is such that failing it will mean death and the party has no other reasonable options...then you're basically playing SAVE OR DIE! And that pretty much always sucks.
As an avid wilderness adventurer in real life I am very much in favor of making the environment and adversary as much as fights with enemies. I enjoy making climbing challenging and simply staying alive in the woods is a challenge.
While it sounds like fun to you (and maybe others), thats something that should be discussed with a group first. Rarely do players like the whole "survival" theme (most want to play Big Damn Heros™, and barely surviving feels a far cry from that). Especially since basic competency at 1st level pretty much defeats all of it. The rules of Pathfinder as presented don't really present a system under which this works without making extensive houserules to the base assumptions of the game.
I'm not sure it's a Save or Die situation, the OP did say that nobody manged to save him, which implied there were some possible rescue attempts.
Also, shouldn't non-combat challenges have possible downsides? Falling to one's death if they 1) don't prepare, 2) fail the skill check, 3) party members can't save them seems similar to a death in combat where the PC 1) isn't prepared, 2) gets critted, 3) party can't save them.
There isn't enough information to even attempt to assign blame to the OP in my opinion. Though it would be good to know if the death was because the party didn't prepare, and wasn't resourceful enough to save him once he slipped, or if it just came down to a d20 climb check or die.
You're right. I may have been a little quick to jump to that conclusion, though his question at the end came across to me as "How can I use the environment to kill my players?"
As far as nobody managed to save him, we don't know if that means no one was in a position to help him or what. We also don't know if the group knew in advance to prepare for this sort of thing or not. Preparation is not always viable. Not everyone carries a climbers kit and pitons when they set out at the beach (or whever they may have started). And if you just happen to run into it, players generally just go along with a situation and try to get through unless given some sort of indication that they have little to no chance of success. This sort of situation is the GM's onus. As GM you set up the situaiton, knowing what options are available to your characters, what is practical courses of action for them to take or reasonable for them to consider/do. While I agree there should be tangible penalties for failing to aduquately "scale the mountain", I don't think that penalty should result in death except for in extreme circumstance. Having a character fall to a lower landing and take 50% of his HP in damage is still a penalty. Slowing down the party if they're in a rush is a penalty. There are plenty of things to do that are better (in my opinion) than allowing a situation to result in character death.
If the reward the party would have gotten would have warranted a death among their ranks, I'd count this like a pit-trap since it seems like a segue into an awesome encounter, in that the BBEG needs to cull the weak.
Speaking of exploration adventures our GM wanted us to be in the wilderness on our own, but realized soon that without civilization there is no reason for us to leave our mud-huts. I suggested we do something like lost but he didn't even want there to be buildings. We ended up with a dungeon building campaign that killed 4 players 4 sessions in, prompting me to leave since apparently the npc mobs had broken stats to compensate for our tank's broken build.