What did Minho find in the maze and why is it significant? (Chapter 12)

Thomas asks if Minho found anything in the Maze. Minho tells Thomas that on any other day that would be the dumbest thing to ask a Runner, but not today. Thomas asks why, but Minho says to wait until Alby returns. Thomas demures but asks Minho to make sure that Alby lets him hear the news too. Minho agrees. Thomas begins to warm to him quickly.

Alby returns with water, which Minho downs thirstily. He tells Alby and Thomas that he found a dead one. “A dead what?”, inquires Alby.

“A dead Griever.” This is unprecedented