Waterfall (たきのぼり,Taki no Bori?, lit. "Waterfall Climb") is Squirtle's Up Special Move. A torrent of water appears that Squirtle "surfs" on top of.
The attack does multiple hits with minor knockback towards the end. It is difficult to interrupt as a recovery move, as the hitbox is large, disjointed, and directly in front of Squirtle. It is also possible to change it to move more vertically or horizontally by tilting it left, right, up or down on the control stick. A big problem is that it can be blocked by an edge as this attack moves diagonally upward.
In the Pokémon games, this move is a Water-type attack, whose characteristics are a power of 80 and an accuracy of 100%, and gained the possibility to make the opponent flinch from Generation IV onwards. From Generation II, it becomes an HM (Hidden Machine), and can be used outside of combat to climb waterfalls to continue the adventure. It started out as HM07, but was changed to HM05 from Generation V onwards, following the demotion back to TM status for Rock Smash, and the removal of Defog (the Sinnoh games)/Whirlpool (the Gold/Silver remakes) from HM status completely.