Water polo is one of the Olympics' most demanding sports - fast and furious from first to last.
Players must tread water throughout
The game consists of four seven-minute periods, during which no quarter is asked or given.
While rough play is punished, most of the players' bodies are submerged underwater out of the view of the referees, so it is difficult to spot foul play.
Holding or pulling people down under water, grabbing, pushing, eye gouging or swimming trunk yanking are all common occurrences.
In between the rule bending, the aim of the game is to score more goals than the opposition.
In Athens, 12 teams will contest the men's competition in two groups of six prior to a convoluted knockout stage.
Group A: Croatia, Hungary, Kazakhstan, Russia, Serbia & Montenegro & USA
Group B: Australia, Egypt, Germany, Greece, Italy & Spain
Group A: Australia, Greece, Italy & Kazakhstan
Group B: Canada, Hungary, Russia & USA
Each sixth-placed team will play the fifth team from the other group, with the winners going on to play the fourth-placed teams to determine who finishes seventh.
Each third and second-placed team will also play off against each other before the winners play the first-placed teams for a place in the final.
The women's tournament consists of eight teams with the winners of each group progressing to the semi-finals where they will play the winners of the play-offs between the second and third-placed teams.
Squads are made up of 13 players with a maximum of seven in the pool at any one time.
The goalkeeper, who guards an area measuring three metres by 90cm, is the only player who can touch the bottom of the pool.
In front of him the team consists of two banks of three players - three guards in defence and two wingers and a centre forward in attack.
The key to success is quick passing and dribbling as teams only have 35 seconds from gaining possession in which to shoot.
If the time expires then the ball passes over to the opposition, as it does if the team in possession commits a foul.
Keeping the ball longer than permitted, using both hands and equally minor infractions are deemed ordinary fouls and result in a free throw.
DID YOU KNOW?
Players can swim up to four miles during a match
Unsportsmanlike conduct and deliberate misconduct results in a major foul and the 20-second dismissal of the guilty party, during which the emphasis is on the opposition to ensure they take full advantage by scoring.
If a player commits three major fouls he will be excluded from the rest of the match and can only be replaced once his third 20-second penalty has been served.
The sheer physicality of the game means fouls occur at regular intervals and a game can last more than an hour in total.
If the scores are level at the end of regular time, two periods of three minutes are played.
If no side has won then a sudden-death period is played, where the first team to score wins the match.