Okay, so I modded in some new metals and stone and stuff. Then, on the embark screen, I tried using site finder, but it just made the game crash.
Then, upon trying the manual site finding, I noticed that things were being repeated, like having granite twice. It also said weird things about the location or the brook or whatever being a soil layer, often repeated a lot. I had a site where the volcano name was mentioned five times as a soil layer. Then I decided to embark at a somewhat random location with a brook (mentioned 4 times as soil).
As I prepared for the journey, I noticed that most of the stones were missing with a value of 0, and the corresponding blocks were just called "rock blocks." Finally, one of the metals I modded in, which had the [DEEP] tag, was appearing with a value of -29 thousand something for all sorts of things like picks, cages, and crossbows. When I finished looking at the weirdness, I embarked.
Everything looked mostly normal, there was a brook, grass, boulders, mountains, but the ground was flashing through every kind of rock that was missing from the embark screen. Also, the walls of the mountain were just called "rock." When I tried to mine, my miners did absolutely nothing.
Then I decided to build some buildings. That was done with the rock I brought with me, but the grass vanished instantly and was replaced by the rainbow ground. Despite the fact that I could survive until the caravan arrived, I abandoned.
What causes this to happen?