The largest anti-submarine warfare exercise hosted in Canadian waters in two decades is now underway off Nova Scotia's coast.

The exercise is called Cutlass Fury. It involves 11 warships and more than 24 aircraft simulating military activities approximately 160 kilometres southeast of Halifax.

Much of the exercise is aimed at detecting and evading submarines.

"As this exercise progresses, it becomes more complex," said Petty Officer Shawn Swinimer, the underwater warfare director onboard HMCS Fredericton.

HMCS Fredericton is one of the 11 ships taking part in the exercise. (The Canadian Press)

"It's close to a real warfare scenario," he said. "It is the world's biggest game of hide-and-seek."

For the next two weeks, ships from Canada, the United States, Britain, France and Spain will criss-cross a vast area of ocean.

From the deck of HMCS Fredericton, at least three large warships loom on the horizon at any given time. Some have helicopters circling overhead, while others practise mid-ocean refuelling.

At one point during the exercises, three helicopters converged on a specific point off the starboard side of HMCS Fredericton. They were hunting a submarine.

CBC's Brett Ruskin joined Canadian military forces as they embark on an NATO anti-submarine warfare exercises off Nova Scotia's coast. 1:02

Finding something you can't see

Anti-submarine warfare rewards strategy and intelligence over brute force.

Often, submarines know a ship's location before the ship detects the sub.

The main point of the exercise is to try and track submarines. (Darren Calabrese/Canadian Press)

Ships can usually outrun subs, but first they must find them. To do this, ships rely on high-powered binoculars, sonar and even helicopters.

Each Canadian frigate is equipped with a Sea King helicopter. The aircraft can be deployed to search the surrounding waters for the sub.

Naval helicopters use sonar by dipping a sensor into the ocean. The aircraft have the advantage of being able to take multiple readings from different locations.

Plus, they can't be hit by a sub's deadly torpedoes.

"Just one of the torpedoes could take out a ship," said Capt. Craig Skjerpen, task group commander for exercise Cutlass Fury.

"It could take out an aircraft carrier."

Military officials say that's why anti-submarine exercises like these are so important.

More subs, more problems

Submarine activity by countries like Russia has ramped up in recent years. Subs are also a relatively inexpensive way for poorer governments to bolster their navy, and are now used by many countries with authoritarian regimes.

Naval experts estimate there are up to 300 submarines operating at all times around the world.

The exercise continues until late next week, finishing up in St. John's. (Submitted by Maritime Forces Atlantic)

During this month's exercise off Canada's coast, commanders have a plan should any of these uninvited foreign submarines drift toward the exercise.

"It does happen and we do have procedures," said Swinimer.

"We'll contact them and then authenticate that they are not an exercise player, and then pass that information on to command," he said.

Swinimer said the exercises are starting with simple search and reconnaissance. As the exercise progresses, it will escalate to full war games with simulated weapons launches.

"It can be stressful from time to time. It's very fast paced," said Swinimer. "But it's not chaotic. Everyone has a set of rules and knows their role."

The exercise continues until late next week, wrapping up in St. John's.