When a friend recently invited Jennifer McDonald to play underwater hockey, she assumed it was a joke.

“I’d never heard of it before, so when he first said it I thought he was crazy,” said McDonald, a 24-year-old student from West Town, who attended her fourth practice on Saturday.

But her friend, Richard Bell, was being perfectly serious.

McDonald may be a newbie. But the Chicago Underwater Hockey Club has been around since the late 1970s and players regularly compete in national and international tournaments for a sport that accommodates all ages and athletic abilities.

The sport traces its roots back to the 1950s, said Maria deCaussin, 50, a Chicago Fire Department battalion chief.

“The British version of the U.S. Navy Seals started playing a game called Octopush,” deCaussin said. “They had eight people in the water, pushing a ball with a stick.”

Today, teams are made up of six people, but the essence of the game is the same: use short wooden sticks to push a three-pound lead puck along the bottom of a pool into a goal. There are two 15-minute halves and the game is non-contact, unlike its ice counterpart. To prevent accidental injuries, players still wear protective gear, such as gloves, a mouth guard and ear guards.

The game starts with the teams at either end of the pool. A player yells, “Sticks up! Go!” and the pool erupts in splashes as the players dive towards the puck sitting in the center of the pool. Occasional snorkels peep up from underwater and fins kick through the surface, but it’s relatively quiet as the action happens at the bottom of the pool.

Players agree that the most challenging part is holding their breath. Although snorkels are worn, they are really to allow players to keep their face underwater to watch the action.

“Holding your breath requires a lot of cardiovascular strength,” said Patty Green, 40, a computer trainer from Albany Park. “You really have to be in shape.”

Since people can’t hold their breath forever, though, teamwork is key, said Jimm Crosby, 50, a teacher from the South Side, who has played underwater hockey for almost 30 years.

“Having no oxygen, you have to play well as a team,” he said. “Even the greatest players, no matter how good they are, no matter how much better they are than the competition, you can’t win without teamwork.”

Likewise, since all players wear fins to help them swim, athletic abilities are generally equalized in the pool.

“I was not a big swimmer, swimming always felt like practice drowning for me,” said Bell, 34, a woodworker from Humboldt Park. “Everyone is more equal underwater, men really don’t have an advantage over women.”

When the players meet for practice, they generally scrimmage by splitting the group in half, Crosby said.

“There are nights when we’re just shocked at the outcome,” he said. “One team just dominates. It’s not because we put disproportionate talent on one team, it’s just because they played better as a team.”

Members of the Chicago club have traveled around the globe to tournaments in England, South Africa and New Zealand. The sport has now spread to about 200 clubs in more than 30 countries.

To represent the United States in the world championships, players try out about a year in advance and the selected team members play in other games together as practice, said Green, who has been on the U.S. woman’s team twice.

The sport has been gaining more interest and there are new countries every year at the world championship, Green said.

Spreading underwater hockey, however, is challenging because most people have never heard of it, said deCaussin, who is the Midwest director of development for USA Underwater Hockey, the sport’s sanctioning organization.

But for those who know about underwater hockey, instant relationships are formed with other players.

“I can go anywhere in the world where they play and I can go pick up a game with them,” deCaussin said. “It’s a pretty tight-knit group.”