More or less something where all of a sudden, you're walking along, and a bunch of people fall out of the sky and die on impact with the ground.
Then, try for a lower y coordinate. Like, maybe y +500. I know a lower y coordinate, something within the atmosphere of the planetary body is possible. I've done it. I actually put my captain's spawn point a few km up, and upon spawn, the captain fell to the ground. My away team spawned on the ground, and the impact from my fall killed my entire away team.
Although, I have heard that there is currently a bug, where there is a problem placing NPC's at anything other than y=0.