MUCH effort is quietly going into the pursuit of what is probably the world's greatest store of fossil fuel—caches of methane, the primary component of natural gas, stored in structures called methane hydrates, or clathrates (a general term for gas molecules trapped by water molecules). Looking just like ice, they are methane molecules trapped within tiny cages of water molecules. They form where temperatures are low and pressures are high, which is to say, on the sea-floor at the continental shelves, and within the permafrost at the Earth's poles.

As with all fossil-fuel resources, it is hard to estimate just how much methane is trapped in clathrates worldwide. But there is a lot. One litre of clathrates can hold more than 150 litres of methane. Numerous deposits have been identified off the coasts of all of the continents. Even a few of the lakes in Central Asia are just frosty enough to support clathrate formation. Some guess that clathrate methane reserves could equal twice the rest of the world's fossil fuel supplies combined.

America's National Energy Technology Laboratory put together a consortium of other government agencies and petroleum companies to drill for clathrates with some success in the Gulf of Mexico; they were promptly hired by India to perform the trick there. A Japanese government collaboration has drilled about 30 wells, with a timeline to start production and distribution of methane from hydrates by 2016. In June China reported having pulled up some first methane-bearing samples from the South China Sea.

All of this might sound like the beginnings of the solution to the world's energy problems. And it may yet be. But, as always, there are some daunting details to sort out first. Many deposits will yield just a fraction of the hoped-for methane, and harvesting even that will be difficult. The little cages of water around the methane are dangerously delicate, so that collection has to take place on the sea-floor. Much work is now under way on adapting conventional drilling equipment for large-scale deep-sea methane recovery.

Clathrates are suspects in a number of geo-crimes great and small. Mixed with sea-floor sediment, they can constitute vast unstable deposits prone to underwater landslides. Such a landslide 8000 years ago in the North Sea created a tsunami that flooded much of coastal Scotland and Norway.

Vast releases of methane from clathrates are widely thought to have played a part in two global temperature spikes that led to mass extinctions about 250m and 55m years ago

And, given their delicate nature, clathrates tend to release their methane bounty during these landslides. Methane is the cleanest of the fossil fuels when burned; but released directly into the atmosphere, it is a “greenhouse gas” significantly more potent than carbon dioxide. Vast releases of methane from clathrates are widely thought to have played a part in two global temperature spikes that led to mass extinctions about 250m and 55m years ago.

Because the icy slush left over after methane removal is less structurally stable than the clathrates, stripping the seafloor of some of its methane might result in frequent landslides that release much more methane. Many clathrate deposits sit atop grand reservoirs of free gas, so that drilling might unleash a methane burp of enormous size, with environmental impacts to match.

One brilliant-sounding idea, now being studied, calls for pumping carbon dioxide into the clathrates. The carbon dioxide would make the clathrates more stable; and, its presence would case them to give up their methane, sequester the carbon dioxide, and let off a little heat that kept the reaction going.

The technological challenge is vast, but no more so than the potential economic rewards. The trick is to get the gas, without the pains.