A bottom slab, stair, or leaves placed on the surface updates solid opaque blocks below it to be lit by the sky light level. This is normal for the block immediately below it, but the effect is propagated downward until an air block is reached. This causes any cave beneath one of these blocks to be lit as if it were exposed to the sky. The effect is reset when something causes light to be recalculated, such as unloading and reloading the chunk, or reset for any single solid block if it is broken and replaced. The effect is not reset by breaking the slab, stair, or leaves, including if you replace them with solid opaque blocks.
This bug makes it possible to temporarily light up a cave by placing slabs or stairs on the surface above it.
Steps to reproduce:
- Find a dark spot in a cave below the surface with no other cave above it.
- Note the coordinates of a block in the dark space.
- On the surface, place a bottom slab, stair, or leaves at the corresponding X/Z coordinates.
- Return to the spot in the cave.
The cave is still dark.
The cave appears lit by skylight at the ceiling block with those X/Z coordinates. If you break the ceiling block, it will still be lit by the block above it. If you replace the broken block, the cave goes dark again.
The fix for MCPE-41273 may have caused this issue.