Walkable Submarine

Minecraft vehicles are single entities you sit on — the game assumes block structures never move, so a submarine with a walkable interior that dives, turns, and carries passengers breaks its most basic physics assumptions.

The first iteration built the block-ship system from scratch for 1.21.11: a helm block assembles connected blocks into a moving ship with collision, seats, torpedoes, sonar pings, an upgrade panel, and a HUD tracking depth, hull, and crush-depth warnings. The second iteration rebuilt the core on Valkyrien Skies for 1.20.1 — a blueprint item places a starter sub, VS makes it a walkable moving ship, and pilot input travels client-to-server where it's applied as physics forces, with hull protection and furnishable interiors.

Game mods need their own test harness — an in-game /submarine smoke-test command plus a script that launches the dev client, drives a disposable world, watches the logs, and collects failure artifacts caught regressions that no unit test could see.