VISBY, Sweden -- Visby harbor on the Swedish island of Gotland is a picture-perfect idyll: the blue Baltic Sea laps against a beautifully maintained quay, and a sparkling white ferry awaits passengers at one end.

But the waters off Gotland are not as clean as they seem on the surface.

In fact, pollution is killing this European sea -- and by spewing untreated waste from a sewer system one activist described as "medieval," the Russian exclave of Kaliningrad is doing more than its part, marine scientists, environmentalists, and European officials say.

"This is one of the world's most polluted oceans," said Fredrik Wulff, a professor of marine systems ecology at Stockholm University and a leading authority on the Baltic Sea. "Because it's an almost closed body of water, everything that's dumped here stays for decades."

Lately, some of the most egregious dumping is being done by Kaliningrad, a little piece of Russia that stands deep inside the EU, bordered by Poland and Lithuania and fronting the Baltic Sea some 300 kilometers south of Gotland.

To the consternation of Sweden and the other EU countries lining the Baltic, Kaliningrad pours a daily dose of some 150,000 cubic meters of raw sewage into the sea.

"It's incomprehensible that Kaliningrad, a city 20 times the size of Visby, is allowed to keep spewing sewage into the Baltic Sea," Peter Landergren, an official in charge of water issues at the Gotland county administration, told RFE/RL at the Visby quay. "Simply pumping the waste from 450,000 residents plus local industry into the Baltic Sea is a marine disaster."

Kaliningrad isn't exactly allowed to dump its sewage into the Baltic Sea -- rather, its neighbors cannot prevent it from doing so.

Not that they haven't tried.

Since 2007, the Northern Dimension Environmental Partnership (NDEP) -- an agency funded by regional governments -- the Swedish International Development Cooperation Agency (SIDA), and the Russian government have been financing a Kaliningrad wastewater treatment plant that was scheduled to be ready for use in 2011.

When 2014 turned to 2015 and no sewage plant was in place, the exasperated donors complained to the exclave's governor, Nikolai Tsukanov -- not for the first time.

"And now, when 90 percent of the plant is completed, Kaliningrad has replaced, without retendering, the contractor with a local company, which will take responsibility for the whole plant," NDEP manager Jaakko Henttonen said last month, speaking from Kaliningrad. "It's like a bad joke."

Tsukanov, who has been governor since September 2010, did not respond to a request for comment.

Declining Fish Stocks

The Baltic Sea's pollution problem is fairly simple. The nitrogen and phosphorus contained in sewage and agricultural waste leads to eutrophication: algae get too much nourishment and take over the ocean, then sink to the seabed where they consume the oxygen on which small fish such as herring depend. That, together with poor fishery management, has led to declining stocks of the ocean's top predators, such as the cod.

By some measures, Kaliningrad is not the biggest offender. Poland's waste -- mainly from agriculture -- accounts for 30 percent of annual nitrogen emissions into the Baltic Sea, and the European Commission took Poland to the European Court of Justice in 2013 over its continued pollution of the sea.

Poland is followed by Sweden at 12 percent and Russia at 11 percent. Per capita, however, Kaliningrad city and its surrounding oblast are a far bigger polluter than Poland or Sweden, as their population of 942,000 pales in comparison to Poland's 38.5 million and Sweden's 9.6 million. Russia is also responsible for more nitrogen emissions than the Baltic former Soviet republics of Lithuania, Latvia, and Estonia.

The ferry business is another major perpetrator, with many of the majestic cruise liners one sees docked in Visby unapologetically emptying their latrines during their journey.

Kaliningrad does the same on a grand scale.

"Kaliningrad, in terms of water ipes and sewers, is a completely medieval city that pours its waste into the gutter. Just as they splashed it out the window in medieval cities, we throw it out not far from Kaliningrad, in just the same way," said Aleksandra Korolyova, a Kaliningrad-based activist with the Russian group Ekozashchita (Environmental Protection). "It's very clearly visible: It's just a black torrent that pours out of the pipe directly into the lagoon, and the lagoon is part of the sea."

Europe's Last Chamber Pot

Poland and other former Soviet-bloc states on the Baltic now boast modern sewage plants, as does St. Petersburg, the Russian city at the eastern end of the sea, where reconstruction of a major sewage plant co-financed by SIDA is also well under way. St. Petersburg has three major water-treatment plants as well as 12 smaller ones; 14 of the 15 meet the Baltic Marine Environment Protection Commission standards and the 15th is currently being upgraded.

Indeed, said Wulff, thanks to Baltic Sea countries' efforts to reduce pollution, the nutrient load has now returned to 1960s levels: poor, but not quite as terrible as a decade ago. Such efforts continue: Visby is building a pipe that will allow ferries to deposit their waste in the Swedish port city's harbor, where it will be treated by the city's sewage plant.

The city formerly known as Konigsberg, meanwhile, is lagging behind -- a charge leveled broadly by Kremlin critics who say Russia has taken a once impressive European hub and let it go to pot since the Soviet Union seized it from Germany near the end of World War II and renamed it Kaliningrad in 1946.

Kaliningrad's sewer system is "very old" and "just cannot cope" with the volume of waste it produces today, Korolyova said.

"When the half-million-strong population of Kaliningrad sits on the toilet, everything...pours into the river or the pipes, and then directly into the sea, in that same natural form," she said.

"So we are probably the last chamber pot in Europe that is dumped into the Baltic Sea."

It's not just the main city that is the problem: Waste from all over the region, whose total population is about 940,000, ends up in the Baltic.

In addition to human waste, Korolyova said, "there is also the industrial sector, car parks, the flood drain system, laundromats, dry cleaners and all that taken together -- it all goes into the sea."

Promised Plant

The new contractor in Russia has now promised that the EU-funded plant will be completed by the end of the year.

"Of course we hope that the schedule will work, but we're skeptical," said Anna Tufvesson, SIDA's water policy specialist, who is handling the Swedish contribution to the Kaliningrad project. "The new arrangement carries large risks, and if there are further delays we'll apply appropriate pressure to make sure a working wastewater treatment plant is installed. If it fails, we will eventually ask for our money back."

NDEP is taking a similar approach. "If there's progress by September, we'll keep funding them. If not, we'll most likely ask them to return the allocated grant following a resolution by the NDEP Assembly," said Henttonen.

The donors will examine the plant's progress again this month.

Having to return its €10 million from NDEP and €15 million from SIDA would, of course, be bad news for Kaliningrad. But environmentalists say the real losers from stalled construction are the Baltic Sea's codfish and herring -- and European officials say Kaliningrad's foot-dragging puts a damper on efforts in other shoreline countries to cut pollution.

"Sweden already has pretty strict rules regarding waste, and here on Gotland we're trying to introduce stricter standards on homeowners and business," said Landergren, the Gotland water official. "But people are reluctant to go along with these rules because they know that 300 kilometers from their front door, a huge city is just pumping waste into the ocean."

Regardless of the outcome in Kaliningrad, there's a glimmer of hope for the busy sea that gave rise to the Hanseatic League, a medieval predecessor to the European Union, and now binds eight of the union's member states together. Scientists are experimenting with ways of pumping oxygen into the seabed.

"It has already been successfully tried in lakes," said marine ecology professor Wulff. "But this is the first-ever attempt at saving large parts of an ocean."

With reporting by Andrei Korolyov in Kaliningrad, Russia