There are a few attacks in the Super Smash Bros. series that cause the bury effect. Characters hit by such attacks will be embedded in the ground. If the target is not on the ground, they will take regular knockback. If the target is already buried, the length of the effect will not be increased or reset.
 List of bury attacks by characters
 List of bury attacks by items
The plunge effect is similar to the bury effect, and in most situations it acts exactly the same. The main difference is that, if the target is standing on a soft platform, they will be hit down through the platform after the "being buried" animation has finished instead of remaining buried.
The only attack to use the plunge effect is that of the Pitfall.