The Great Bear Rainforest is world renowned for its grizzly bears and towering cedars, but in a secret little watershed near Bella Bella on the central coast it is also being distinguished as the place where salmon swim through mountains.

“It’s spectacular, something out of a Tolkien novel,” Ian McAllister of Pacific Wild says of subterranean passageways used by salmon to reach their spawning grounds. “The salmon make it into the cave and then the stream just disappears into the mountainside. I thought, ‘what’s going on in there?’”

McAllister and his colleagues donned scuba gear and unravelled rope this summer to follow the slow-moving stream close to 100 metres into the marble rock before turning back. “I thought we’d go through water then into a dry cave, but it’s all underwater, a labyrinth of marble tunnels and caverns,” he said in an interview. “What’s really amazing is we found all these coho salmon hiding away in the back of this cave in the side of this mountain. Remarkable.”

About one kilometre upstream from its estuary on the Pacific Ocean, the stream flows beneath a fantastic natural bridge forged by erosion. About 50 metres later, it forms a six-metre-deep pool before disappearing into the mountainside of marble.

“It feels like you’re diving through a marble church in Europe or something — big domes of marble on the roof that have been chiselled over thousands of years,” McAllister explained.

Marble is a form of limestone karst, perhaps the most fragile landscape on Earth.

The limestone in this cave system originated hundreds of million of years ago as marine reef and lagoon deposits near the equator, rafting northward aboard ocean plates that eventually collided with the continent.

Karst landscapes are formed by naturally acidic water seeping through the subsurface calcium carbonate rock. This process can form caves or caverns, passageways, and fantastic but easily damaged speleothems such as columns, curtains, flowstone, soda straws, stalactites and stalagmites.

Limestone makes for more productive rainforests, draining away extensive rainfall while the dissolved cracks in the bedrock give tree roots a good foothold against powerful winter winds. Karst also reduces the acidity of rainfall, providing improved habitat for aquatic life, including resident and migratory fish.

Navigating the murky, water-filled passageways proved to be an intimidating experience in which salmon went bump in the night and flashlights provided the only light. “All of a sudden one would come out of the dark and bump right into you,” McAllister said.

“Can you imagine what that feels like? Your imagination is already going wild. You’re in the mountain and it’s pitch back and your imagination starts to get ahead of you. Of all the crazy things I’ve done, this one had my heart in my mouth the most.”

To date, McAllister has made four forays into the passageways. Cutthroat trout were also documented. Bears are known to frequent an archway above the pool to feed on salmon as they swim though a boulder garden before scooting into the mountain.

Ropes were fed out to divers during the expedition so they could get back safely through the network of tunnels. “If you lost the ropes, you’d probably never find your way out again,” McAllister allowed.

Farther upstream, about 200 metres from the pool, the water emerges from the mountain to flow along the surface and serve as spawning habitat. Whether the salmon also spawn in the cave system itself remains to be determined. “There’s a lot more work to be done. The Bella Bella area is rich with karst.”