The baby Zombie Pigmen don't sink into the blocks, so, it only works with adults.
How to produce:
- Spawn a Zombie Pigman
- Put it on a boat
- Use the boat to move around
- Get out from the boat
- Spawn an another Zombie Pigman on the boat
- Push the boat
- Hit the two Zombie Pigmen
- Break the boat.
Result: The two Zombie Pigmen sink into the block.
Note: The Zombie Pigmen sink in one block, but they can't get out.