When you are below the trapdoor,when you open it,you'll swim in land.It's useful for mining 😃
Same thing happens in when you are in trapdoor and when you close it.
Environment
System : Windows 10
Java Version "1.8.0_201"
Java(TM) SE Runtime Environment (build 1.8.0_201-b09)
Minecraft Version : 1.14.2
Duplicate of MC-147612