Off the coast of Canada, you won't see coral reefs — you'll see reefs made of actual glass

You know coral reefs, but have you heard of glass reefs?

Jacqueline Kehoe Creator•Follow
Updated Mon, October 5, 2026


A digital reconstruction of the Anji Biota, a diverse glass sponge reef from the Ordovician of China (Wikimedia Commons)


When most of us picture a reef, we picture the tropics: bathtub-warm turquoise water, clownfish, and coral in every color of the crayon box.

But off the coast of British Columbia, hundreds of feet down in cold, dark water, there's another kind of reef entirely. It's built by animals with skeletons made of glass. For decades, scientists thought reefs like these had vanished from Earth tens of millions of years ago.

They were wrong. Welcome to what conservationists call the "Sea of Glass."

What are glass reefs?

Glass reefs, or glass sponge reefs to use their full name, are built by glass sponges. These are deep-sea animals whose skeletons are made of silica: silicon dioxide, essentially the same stuff as window glass. The difference is that the sponges make it themselves.

And yes, sponges are animals. Glass sponges are rigid, lacy, and startlingly delicate. They feed by pumping seawater through their bodies and filtering out bacteria, and they're very good at it. One University of British Columbia study found they remove nearly 80% of the microbes and particles from the water passing through them.

Only a few species build glass reefs, and they do it the same way corals do. New sponges grow on the skeletons of old ones, generation after generation, adding just a few centimeters a year.

Given enough time, that adds up. The biggest known reef complex, in B.C.'s Hecate Strait, has been growing for more than 9,000 years. Parts of it stand around 80 feet tall, roughly the height of an eight-story building. It covers an area about the size of Manhattan, some 650 feet below the surface.

The discovery of glass reefs stunned scientists

During the age of dinosaurs, glass sponge reefs were everywhere. They stretched across the Tethys, an ancient ocean, and may have been the largest reef systems in Earth's history. Their fossilized remains still form rock formations across Spain, France, Germany and Romania.

But living glass reefs seemed to have disappeared around 40 million years ago. As far as science knew, they existed only as fossils.

Then, in the mid-1980s, Geological Survey of Canada scientists were mapping the seafloor off northern B.C. and kept finding strange mounds and ridges. Kim Conway, one of the geologists on those expeditions, has said it took a couple of years to figure out what they were. Every one of those features turned out to be a reef built by the same group of sponges. The discovery was confirmed in 1987.

One researcher compared it to finding a herd of dinosaurs still roaming on land. (Wow.)
Why glass reefs are so rare, and how you can see one

Glass sponges live in oceans all over the world. But they build reefs almost exclusively in the Pacific Northwest, overwhelmingly off B.C.

Scientists think the recipe is very specific. The sponges need cold water, doing best at about 43 to 48 degrees Fahrenheit. They also need plenty of nutrients and lots of dissolved silica, which rivers wash down from the region's mountains into the sea.

The reefs are also heartbreakingly fragile. Bottom-trawling nets and prawn traps can shatter structures that took thousands of years to build. Stirred-up sediment can clog the sponges' filter-feeding systems. Warming, acidifying oceans are a growing worry, too: a UBC lab study found both conditions weakened the sponges' skeletons and their ability to feed.

Canada made the Hecate Strait and Queen Charlotte Sound reefs a marine protected area in 2017. It has also closed a number of smaller reefs to bottom-contact fishing, but many reefs still lack permanent protection.

The good news is you don't need a submarine to see one. Howe Sound, a fjord about an hour northwest of Vancouver, is believed to be the only place in the world where glass sponge reefs grow shallow enough for recreational scuba divers. The reef off Gambier Island's Halkett Point starts around 80 feet down; dive charters out of Horseshoe Bay take experienced divers there.

Not a diver? Same. The Canadian Parks and Wilderness Society's B.C. chapter has an online "Sea of Glass" story map. There's also a documentary, "Moonless Oasis," about Howe Sound's reefs.

However you see them, it's worth it. It isn't every day you get a glimpse of a world that was supposed to have ended 40 million years ago.

If you love nature as much as I do, I'd love if you could follow me on Yahoo Creators for more articles on our wild world. This shows the Yahoo team what their readers want — and the more brands that support the planet, the better.

Jacqueline Kehoe

Jacqueline Kehoe CreatorJacqueline is a travel, nature, and science writer based in Wisconsin. Follow her work on Yahoo Creators—or find it in the wild at publications like National Geographic, Smithsonian, Travel + Leisure, and more.