They're called "dead zones" - patches of our ocean where oxygen plummets to levels so low that many animals passing through them suffocate and die.
In just the past 50 years, the amount of water in the open ocean with zero oxygen has quadrupled, while low-oxygen sites closer to coasts have increased tenfold.
The picture could only worsen if the world doesn't rein in climate change and nutrient pollution.
And, despite its relative isolation on the planet, our own country isn't immune to their threat.
"Oxygen is fundamental to life in the oceans," said Dr Denise Breitburg, a marine ecologist at the Smithsonian Environmental Research Centre in the US, and lead author of a just-published study.
The decline in ocean oxygen ranked among the most serious effects of human activities on the Earth's environment.
"It's a tremendous loss to all the support services that rely on recreation and tourism, hotels and restaurants and taxi drivers and everything else," Levin said.
"The reverberations of unhealthy ecosystems in the ocean can be extensive."
The new study, just published in the major journal Science, was authored by a team of scientists from the Global Ocean Oxygen Network (GO2NE), a new working group by the United Nation's Intergovernmental Oceanographic Commission.
It was the first to take such a sweeping look at the causes, consequences and solutions to low oxygen worldwide, in both the open ocean and coastal waters.
"Approximately half of the oxygen on Earth comes from the ocean," said Vladimir Ryabinin, executive secretary of the International Oceanographic Commission that formed the GO2NE group.
"However, combined effects of nutrient loading and climate change are greatly increasing the number and size of 'dead zones' in the open ocean and coastal waters, where oxygen is too low to support most marine life."
In areas traditionally called "dead zones" like those in Chesapeake Bay in the US and the Gulf of Mexico, oxygen levels are so low that marine life can die.
As fish avoid these zones, their habitats shrink and they become more vulnerable to predators or fishing.
But the problem stretched far beyond "dead zones", the authors pointed out, as even smaller oxygen declines could stunt growth in animals, hinder reproduction and lead to disease or even death.
It also could trigger the release of dangerous chemicals such as nitrous oxide, a greenhouse gas up to 300 times more powerful than carbon dioxide, and toxic hydrogen sulfide.
While some animals could thrive in dead zones, overall biodiversity falls.
In open ocean dead zones, climate change was the key culprit, with warming surface waters making it harder for oxygen to reach the ocean interior.
Further, as the ocean as a whole became warmer, it held less oxygen.
In coastal waters, excess nutrient pollution from land created algal blooms, which drain oxygen as they die and decompose.
In an unfortunate twist, animals also needed more oxygen in warmer waters, even as it was disappearing.
People's livelihoods were also on the line, the scientists reported, especially in developing nations.
Smaller, artisanal fisheries may be unable to relocate when low oxygen destroys their harvests or forces fish to move elsewhere.
In the Philippines, fish kills in a single town's aquaculture pens cost more than $10 million.
Coral reefs, a key tourism attraction in many countries, also could waste away without enough oxygen.
NEW ZEALAND'S 'DEAD ZONES'
Some fiords of New Zealand were home to dead zones, but these tended to be naturally occurring, University of Auckland marine scientist Professor Simon Thrush said.
Elsewhere, New Zealand had problems with prolific seaweed growth in some harbours that could cause localised and temporary low oxygen, and there had also been cases of seafloor algal blooms temporarily fuelling low oxygen zones.
Thrush, head of the university's Institute of Marine Science, explained there were essentially two processes that led to the formation of dead zones.
The first involved how much water was mixed and moved around, which was why fiords came to have them.
"Fiords are deep and have a shallow sill that limits water exchange with the open coast, Thrush said.
"Often the water in the fiord will stratify due to differences in temperature and salinity, further isolating the water deep in the fiord."
As organic matter slowly sank to the bottom, it was consumed by organisms that also consumed oxygen.
"Because of the low water exchange, eventually, over months to years, a dead zone will develop."
The second factor related to how much organic material was available in the water, or on the seabed to be consumed by organisms.
"When the overlaying waters are highly productive or there are large blooms of seaweeds there will be more organic material sinking to the seafloor than can be processed by the microbes and animals."
All the oxygen was used up, causing the dead zone.
This was the process of eutrophication, often generated by too much nutrient running off the land, and that linked to the prominent dead zones in Chesapeake Bay, the Baltic Sea or off the coast of China.
Thrush said both processes were important and inextricably linked.
What did the future hold for New Zealand?
"The strong tides and currents around most of New Zealand mean we have not, so far, had too much of a problem.
"But we are allowing more and more nutrients to run into our coasts, harbours and estuaries - this is beginning to be a problem in some places."
It was particularly a problem where high nutrient loads combined with relatively stable and poorly mixed water bodies clear enough to allow plants to grow.
"Climate change can increase the risk of dead zones, with more rain bringing more nutrient runoff and increased surface ocean temperatures increasing the potential for thermal stratification in the water column."
Dead zones - or temporary ones at least - could well become more common around our coasts, he said.
"As the name implies dead zones are something we do not want to see, it would be a very clear message that we have really badly treated our coastal zone.
"Apart from killing all of the animals, the sediments will start to release hydrogen sulphides and climate warming gases such as methane."
WHAT CAN BE DONE?
The authors of the new paper suggested a three-pronged approach to tackle the problem.
The first, obviously, was addressing the causes of nutrient pollution and climate change, through designing better septic systems, stemming pollution, and cutting fossil fuel emissions behind the wider problem of global warming.
Secondly, they called for protection for vulnerable marine life, which could involve creating marine protected areas or no-catch zones in areas animals use to escape low oxygen, or switching to fish that are not as threatened by falling oxygen levels.
Finally, low-oxygen tracking could be improved worldwide.
While scientists had a decent grasp of how much oxygen the ocean could lose in the future, they didn't know exactly where those low-oxygen zones will be.
Enhanced monitoring, especially in developing countries, and numerical models could help pinpoint which places are most at risk and determine the most effective solutions.
"This is a problem we can solve," Breitburg said.
"Halting climate change requires a global effort, but even local actions can help with nutrient-driven oxygen decline."
Breitburg pointed to the ongoing recovery of Chesapeake Bay, where nitrogen pollution had dropped 24 per cent since its peak thanks to better sewage treatment, better farming practices and successful laws like the US Clean Air Act.
While some low-oxygen zones persisted, the area of the Chesapeake with zero oxygen has almost disappeared.
"Tackling climate change may seem more daunting," she added, "but doing it is critical for stemming the decline of oxygen in our oceans, and for nearly every aspect of life on our planet."