Villages should occasionally have mineshafts underneath

Villages should very rarely spawn with an entrance to an abandoned mineshaft underneath it. Currently, the only good way of finding abandoned mineshafts are either go to the quite rare mesa biome, or go spelunking in the massive caves in hopes of one sticking out. 

The mineshaft entrance would look like some sort of caged structure, housing some gravel. This gravel would be floating on top of a very long hole to the mineshaft, so if the player tried to mine it the gravel would fall and potentially bring the player along with it. From here, the player is free to explore the mineshaft.