Mole Mario

From the Super Mario Wiki
Mole Mario in action.

Mole Mario is a Bros. move in Mario & Luigi: Dream Team. It is taught by two Shelltops working for Britta in Dozing Sands along with Mini-Mario. When Luigi hits Mario from behind with his hammer above soft ground (like grass, sand, etc), Mario will be buried into the ground. If the two brothers are above hard ground (like metal or rock), Mini-Mario will be used instead. While in this state, Mario can move underground, allowing him to pass under obstacles. If Mario passes through an obstacle in which Luigi cannot follow, he will be separated. Mole Mario can also be used to find Beans hidden in underground spots marked with an X. Once Mario has emerged from the ground, he cannot go underground again without Luigi's help.

This move is nearly identical to Luigi Dunk from Mario & Luigi: Superstar Saga and its remake.