This may seem impossible to those that have tried, but here’s how to ACTUALLY do it. You first need to get a fuel tank, then go to the fuel section. Instead of putting your normal values, put NEGATIVE values. What this will do is make the block have negative weight. If you spawn yourself with negative weight, you’ll glitch into the ground.

I haven’t gone through thorough testing, so I have no clue what this does to flight mechanics, nor if it has any use. Have fun!