It is mainly found by jumping and bumping into [!]-blocks.
It is first mentioned by a sign in Lethal Lava Land, where the Koopa Shell is needed to collect all coins and enables the player to reach farther locations of the level without losing health.
The Koopa Shell is only found in a few select worlds.
Using it enables Mario to surf on hazardous surfaces like lava or acid and, of cause, harmless water. He can also use it to run over various kinds of minor foes like Goombas or Boos.
More content will be added soon. Please be patient.