The acid Samantha believes killed her daughter, the loss that breaks her open in this chapter.
Samantha tells Carl she thinks her child is dead — she thinks she died in the acid. She felt her, and now she does not feel her anymore, and she holds herself responsible for not going to save her sooner.
Carl asks whether she is sure the girl was actually down here at all. Samantha shakes her head sadly: she was here, and Samantha felt her. She then assigns blame to Tish and threatens to nuke the college over it.
The offstage disaster that killed Samantha's daughter and drives her grief.