Sharking is the act of going underneath a stage to attack opponents from below. Its name directly references sharks, which swim close enough to the surface of the water so that their dorsal fins are seen overhead. It is best performed by characters with multiple jumps such as Meta Knight and Kirby, who will be able to recover after performing sharking. It can only be performed on stages containing either semisoft platforms (those that can be traveled upward through, but not downward through), or thin hard platforms (though the latter is more risky for the attacker). Notable examples of the former include Delfino Plaza and Halberd, while examples of the latter include Jungle Japes. The principle behind sharking is that while underneath the stage, a character can attack and/or damage shields relatively safely, while their opponent has few options to counterattack due to being unable to pass through the platform. However, an experienced player can perform a meteor smash on a sharker, which if timed accurately usually results in an immediate KO.
Sharking has historically been a legitimate if risky strategy, as while it allows attack without much chance of counterattack, it is easy to make a mistake and self-destruct. This is more true in SSB64 and Melee, due to their faster physics and falling speed in general compared to other games. In Brawl, Meta Knight in particular can shark with ease on many stages, thanks to a combination of a very fast and disjointed up aerial, plus a choice of four quick recovery moves. This for many has driven sharking into an area considered cheap or even broken on certain stages (though usually only when performed by Meta Knight, perhaps a testament to how some would consider the character himself broken). Sharking is commonly combined with planking to allow the sharking character to restore their jumps and be minimally vulnerable to attacks, though this latter technique is no longer possible in Smash 4 due to its new edge mechanics eliminating the viability of planking.
The technique has been mentioned on the official website for SSB64 under a name that can be translated as "mole tactics".
Characters that can notably perform sharking
Stages that allow sharking
*Indicates that the stage has thin hard platforms instead of semisoft platforms