Instant obsidian farm

ianxofour's instant obsidian farm, and still the standard answer for obsidian years after release. A trapped wither is held under the end portal so its black skulls keep destroying the portal's obsidian, which the game instantly regenerates, giving a stack every seven seconds from less than a chest of materials. It needs three wither skeleton skulls and a stronghold portal room, so it is not an early game build, but nothing else comes close for the cost.

• Raise the black skull blocking wall by about 4 blocks from 1.21.1 onward. Skulls now travel higher out of the bubble column and miss it, and they pile up until the game lags or crashes.
• From 1.20.2 the snow golem sits differently in the boat and the build needs the adjustment linked in the description, including a sticky piston under the soul sand.
• Use a normal boat, not a bamboo raft. With a raft the snowballs will not pass through the portal and the farm does nothing.
• The chunk loader, the end portal and the junk item clock all have to be in the same chunk. The portal room does not always generate in a chunk of its own, so turn chunk borders on before you start.
• Put a seed on the upper pressure plate before the lower one, otherwise the dispenser stops feeding after the first cycle.
• Give it a few minutes before deciding it is broken. The despawn timer has to run first, and most reports of it failing turn out to be impatience.