Off the remote eastern tip of Papua New Guinea a natural phenomenon offers an alarming glimpse into the future of the oceans, as increasing concentrations of CO2 in the atmosphere make sea water more acidic.
Streams of volcanic CO2 bubbles emerge from deep under the seabed here, like a giant jacuzzi.
As the bubbles of carbon dioxide dissolve into the water, carbonic acid is formed.
The site hints at the possible fate of the world's seas as 24 million tonnes of CO2 from industrial society is absorbed every day into the sea.
Humans are turning whole oceans more acidic. We're changing ocean chemistry faster than it's changed for tens - maybe hundreds - of millions of years.
Our Newsnight team joined an international research boat on an expedition to the far tip of Papua New Guinea.
I tug on my wetsuit, 3mm thick. I'm about to see the effects of CO2 for myself. They say I only need a stinger suit - a hooded nylon onesie - to shield me from the deadly box jellyfish. But I feel the cold.
I flop into the lukewarm water from the platform at the back of the boat, and fin slowly towards the volcanic CO2 vents.
The sea bed is a wondrous sight. The afternoon sun illuminates the bubbles as they race towards the surface, encasing them in globules of radiant light.
It's like swimming in a sea of lemonade.
I run a hand through a column of bubbles as it shoots from the sand, half expecting it to hurt. It doesn't harm me, of course - the bubbles are only CO2. But their acidity is inarguably hurting the local sea life.
Only tough old boulder corals can survive here. The beautiful branching corals which adorn a nearby uncontaminated reef can't cope.
It's a huge loss because the branching corals play a vital role in the reef ecosystem, protecting the young fish needed to help feed a hungry world population.
Research at the volcanic vents shows that between 30-50% of coral types won't be able to cope with the CO2 levels expected for the world's oceans this century.
The lead scientist, Katharina Fabricius from the Australian Institute of Marine Science, tells me: "There will be winners and losers as ocean acidity increases. Seaweed and seagrass are thriving under higher CO2 levels. But many species lose out.
"We are very concerned because the baby corals find it very hard to survive in high CO2 so reefs won't be able to repair themselves. It's very, very serious."
Our cameras capture one experiment revealing a startling disparity in the number of species between the normal CO2 area and the high CO2 vents.
There's no comparable place to assess how reef creatures are affected by increasing CO2 so there's sharp competition for places on the research vessel, The Chertan. It's just 18m (59ft) long and while there are nine scientists there are only seven beds. Volunteers sleep on the floor.
It's a fast-expanding area of science, and Fabricius is one of several researchers working in laboratories to see how creatures deal with high CO2 and the elevated temperatures predicted to accompany it.
Terry Hughes, director of the Centre for Reef Studies at James Cook University in Australia says acidification is the latest threat to reefs.
"We've already lost a third of coral reefs thanks primarily to pollution and overfishing - both are accelerating. Now there's the added impost (sic) of global warming and in future, ocean acidification.
"I'm very worried about acidification. Some coral species will substitute for others, but if you lose table corals and tall branching corals, most of nooks and crannies - the hiding places for juvenile fish - will disappear. And it'll directly affect humans being because fish stocks will be affected."
Research on acidification is now spreading from corals to fish. One group of scientists at the university is chasing fish in a barrel to see if their athleticism is compromised by water that could be 170% more acidic than pre-industrial times by the century's end (although still alkaline overall).
Already tests show acidification makes some fish lose their sense of smell and behave recklessly in the presence of predators.
A draft UN report also warns that mass extinctions happened in the past, when CO2 levels changed more slowly than they are changing now.
"The changes we're making are irreversible for tens of thousands of years," Fabricius tells me as the sun sets over the dugout canoes heading home after a day's fishing.
"We can protect reefs from over-fishing and local pollution if there's a will. But with the atmosphere and oceans it's completely different - there's nothing to remove the effects of CO2 from the system. It's terrible."
Follow Roger on Twitter @RHarrabin
Watch Roger's report on Newsnight at 22:30 GMT on BBC Two on Thursday 27 March (camera Matt Leiper, producer Nick Blakemore)