Bubbles the size of basketballs

The location of the methane bubbles, 650m down, took submersibles an hour to reach. They were visible from the ship at the surface – a plume of small bubbles showing up on the echo-sounder that scans the sea below for activity. After an hour of searching for the bubbles at depth, something suddenly shot up from the sea bed. The team had found what they were looking for. Producer Orla Doherty shares how it felt.

“It was an extraordinary sight – it almost literally took our breath away. What had been an empty desert plain suddenly turned into an erupting seabed. It was really hard to take it in, it felt almost as if we had landed on another planet. But we knew we needed to get the cameras rolling on it immediately. We didn’t know how long the eruption was going to last.”

Methane bubbles are formed by the decomposition and compression of tiny particles of organic matter that rain down from the surface of the ocean, smothering the seabed in a fine carpet of oozing goo. The mud volcanoes form where deeply sourced fluids move up through the layers of sediment. The flow is thought to be slow and steady, but from time to time, as the pressure builds up, there are huge eruptions.

What we don’t know is what causes this spectacular sight. A number of factors could be involved, including movements along a fault line - salt tectonics drive a lot of seepage in the Gulf of Mexico. This could generate an expulsive event; turbidity flows, which cause changes in pressure fields; or simply an accumulation of pressure which has to be released, much like a pressure-cooker’s release valve...the mud volcano is the release for the deep ocean floor.

“We named this dive site ‘War of the Worlds’. The next day, we dived it again – the seabed had calmed once more, returning to an apparently empty desert plain. There wasn’t a single bubble of methane to be seen. What we had seen, we never saw again for the rest of the expedition” says Doherty.

This landscape could change in the decades to come as the climate changes. Increases in deep-water temperature of only 3°C could destabilize the delicate structure of methane hydrate deposits that occur on the continental slope. This would release methane that may reach the atmosphere and as methane is a potent greenhouse gas there would be a positive feedback to global climate.

These observations made during filming of Blue Planet II in the Gulf of Mexico will lead to two potential papers written by Dr Samantha Joye, a Professor of Marine Sciences: one which will report the deep water temperature trends in the Gulf of Mexico deep which look to be increasing and another, which models the temperature data to predict the stability of methane deposits on the seafloor and the implications for our climate and the animals that live in the deep sea. Joye explains:

“I have been diving in submersibles to conduct research in the Gulf of Mexico – and elsewhere – for over 20 years and I’ve never seen anything like the eruption we captured at the Mud Volcano. Diving there before, we had documented walls of tiny champagne sized bubbles outlining the edge of the mud volcano’s crater, but I’d never such enormous bubbles, or the density and rate of large bubble release. The scene left me breathless.

"We documented vigorous gas discharge – though not the large bubbles – at other sites and observed a surprising absence of surface-breaching gas hydrate at sites where they had always been present before. Now we are going back and assembling the physical data from various research teams collected during the past 25+ years across the Gulf’s Northern shelf and slope to assess whether the median and variance in bottom water temperature has changed.

"The Gulf of Mexico is home to enormous reserves of shallow methane hydrate, hence documenting changes in temperatures in the past, now, and into the future, is critical,” says Joye.

#OurBluePlanet is a collaboration between BBC Earth and Alucia Productions.