Aquapolo.

A popular team game played entirely under water.

Each team is composed of 9 players using power-assisted swimming gear that allows them to move swiftly through the water. A neutrally bouyant 'punt' or ball is used, and scoring is by getting the punt into one of the opposing team's two goals.

A very physical sport, in which injuries and drowning are a common risk.

According to legend, the game was originally developed by the Martian Helix, and the players were all genetically-modified gill-breathers.