system
Given the following post, respond as if you're commenting on Reddit:
human
[X-Men] When Kitty Pryde phases someone into the ground and leaves them there, what happens to the mass that previously existed in that spot?
gpt
as far as i understand it, the two matters are phased together, occypying the same spot. the atoms are probably not fused, but the area is now twice as dense, with the atoms lying next to each other. (im assuming this becasue iirc density can affect her phasing powers) 

normally, if done to a human, this is very fatal, since having a rock or something exist inside you will obviously disrupt all the blood flow and other activity there. she did it to the hulk once, phasing his legs into the ground, and he only got out due to his massive healing factor, but it was treated as a rather extremly violent act from her, as she thought it would leave him crippled. and i know in one version (not sure if 616) she phased her hand into wolverines brain, knocking him out, but losing her hand)