Back in Season 5, shortly after Hardhome, Jon Snow and Samwell Tarly had an honest conversation about the Army of the Dead, and what they’d need to defeat them. Sam asked about a lost bag of dragonglass, and Jon made the point that one bag wouldn’t matter much — they’d need an entire mountain to win.
Now, here we are in Season 7 and what has Sam found in a Citadel book? A map of Dragonstone showing just what Jon was dreaming of: a mountain of dragonglass. Check out the page below.