Sydney Harbor is one of the world’s most picturesque seaports, renowned for both its natural beauty and iconic architecture. Yet after two centuries as the front porch of a major metropolis, it also hides an ugly secret below the surface: Garbage now blankets the seabed around Sydney, some of it nearly as old as the city itself.

All this trash causes a variety of local problems, environmentalists say, such as leaching toxic chemicals into seawater or tempting marine animals to eat tiny bits of plastic. And it may feed a much broader environmental threat, too, since some of the debris washes out to sea and could ultimately join the Great Pacific Garbage Patch.

But thanks to a coalition of local divers and environmentalists, Sydney Harbor’s garbage problem is undergoing a rare — albeit slow — change of fortune. As the BBC and Yahoo News report, Sydney-area divers have begun volunteering their time and talents to manually remove trash from the harbor floor, tackling a massive problem that has festered behind the scenes for nearly two centuries.

“The stuff on the bottom has been accumulating for 200 years, and it’s only now we’re really trying to pull it out in any sort of quantity,” local diver Dave Thomas tells Australia’s Seven News. “It’s been out of sight and out of mind.”

The effort is led by two Sydney-based conservation groups: Eco Divers, which conducts marine research and rehab missions via scuba diving, and the Two Hands Project, which rallies volunteers for beach cleanups and other environmental ventures. As Thomas tells Seven News, this kind of work comes naturally to people who spend a lot of time in the ocean. “As a diver you see this stuff, and if it was on land you would be disgusted,” he says, “and you would do something about it.”

Loose garbage remains a problem in urban areas worldwide, but coastal cities face unique challenges: Their own trash can hide offshore, either lurking in place or washing into ocean gyres, and they can also receive transoceanic trash from other cities, such as Japan’s tsunami debris now reaching U.S. shores. Sydney’s waste woes are especially severe, both for the local environment and for the Pacific at large, since ocean currents can carry debris to New Zealand and beyond, potentially feeding garbage patches in the North Pacific. The local government hasn’t ignored the issue, though — it employs a harbor cleanup team, for example, which began boosting its productivity in the 1990s by using prison inmates to help it collect trash.

But the prison-labor program was nixed in late 2010, and as the Sydney Morning Herald reported earlier this year, trash removal from the harbor has fallen since then — even as heavy rains washed more debris out to sea. State cleanup crews collected just 2,284 cubic meters (80,658 cubic feet) of waste in Sydney Harbor during the last financial year, the paper reported, down 18 percent from the year before. This was partly due to other factors, namely the temporary loss of a key cleanup vessel, but losing so much labor has likely also played a role.

“One can draw the conclusion that there would be more litter in the harbor,” Peter McLean, the New South Wales director of Keep Australia Beautiful, told the Herald in January. “I hate to see programs like this not continue in some form. It would certainly be very detrimental.” Yet despite such setbacks, the NSW government maintains its stated goal of having Australia’s lowest per capita litter count by 2016.

Eco Divers’ and Two Hands’ cleanup efforts could help with that, but it won’t be easy. As the BBC reports, some of Sydney’s marine debris is big and unwieldy, such as old bicycles and bathtubs, and even smaller objects like plastic bags, bottles and fishing lures are labor-intensive to remove by hand. And while it’s hard enough to collect two centuries’ worth of trash from the bottom of a busy harbor, more than 900 metric tons of garbage also washes back onto local beaches every year, much of it becoming trapped on rocky shores, Seven News reports. Sydney cleanup crews cover some 12,000 acres of waterways and 170 miles of shoreline, according to the Herald.

It is a daunting task, but as Two Hands’ Dean Cropp tells the BBC, any progress is worth the trouble. “Unless someone cleans it up, it could be there for years,” he says. “It could be there for hundreds of years — doing damage the whole time.”