The submerge glitch is a means of porting grass/rock Surfing glitch over to non-Spanish/Italian Pokémon Red and Blue versions with arbitrary code execution using a simple modification to RAM.
While Surfing on water, the modification of D535 (grass tile) (or D534 in Yellow) to 0x14 (the same ID as a water tile) will trick the game into loading the water encounter table with the grass encounter rate. Hence, encounters like in the grass/rock Surfing glitch are possible provided that a Trainer was encountered beforehand, including usually uncommon (due to its inaccessibility with D058) 3TRAINERPOKé₽ battles. However, after each encounter D535/D534 will be reset, hence the submerge glitch may have to be repeated.
While active, the game will place the player sprite inside the grass as expected, but because the grass was defined as a water tile it will give the illusion of the player being partially submerged in the water, hence the name of the glitch.
The glitch while currently arbitrary code execution only, might be available with a carefully prepared extended Super Glitch.
An unintended encounter sourced from last opposing Trainer data
- Torchickens (documentation)