Minecraft wiki entry for Mineshaft, associated text: Starting point: a 10×10 parlor, with an arched ceiling and one to four exits in each direction.
Corridors: some 3×3 tunnels and junctions are supported by planks and fences. Some places are decorated with torches and cobwebs. On long corridors, these supports are placed four blocks away from each other while some have missing planks which may be filled up with cobwebs or blocks of stone jutting out from the top. The tunnels are connected by incomplete rail systems on the ground, on which minecarts with chests generate uncommonly. Cobwebs do not generate if they would not be connected to solid blocks, while corridors generate with log pillars, chains and fences for support if they are in mid-air.
Crossings: dual-floor, 5x5 intersections supported by wooden columns.
Staircases: diagonal tunnels, stair blocks are not used.
Abandoned passageways: accessory corridors with cobwebs concealing cave spider spawners.