The note can be found in Carleton Mine, in the room where Lou is found.
Of course, I'll use gravity! Nothing can stop gravity. It's gravity!
The stone pillars in this cavern are load-bearing. When my explosives detonate, the ceiling will collapse. I'll be buried under a ton of rubble. If I don't die in the explosion, at least I'll never get out.
This is going to work, dammit. I'm a demolitions expert. Why didn't I try this from the start?