system
Given the following post, respond as if you're commenting on Reddit:
human
If you were to make a hole all the way through to the mantle, what would happen?

Lets assume you have a magic heat-proof drill of a ludicrously powerful shaped charge or laser or something that will make a hole, say, a couple of meters wide straight down all the way through the crust where I am in the UK. 

Would you get an instant volcano? Would the results be different if it were under the ocean, where the crust is thinner (or have I got that backwards?). I suspect the planet doesn't just deflate like a balloon...
gpt
What happens when you drill a hole in the ice on a lake? The water below the ice rises up through the hole to just below the top. This is because the ice is floating on the water, so the water below the ice is under pressure and is forced up the hole until the weight of the water column is equal to the weight of the ice around it. 

Same if you managed to drill through the crust. The exact results depend on how you're keeping the hole open and how dense the crustal rock is compared to the magma below. 

You could also get some interesting behavior as dissolved gases are released (interesting as in being close would not be a good plan).