sure to drill for oil wherever it may be found — including ‘frontier’ areas such as the Arctic Ocean. With an estimated 80 billion barrels, or 4% of all the oil that could be recovered by conventional means, Arctic waters loom large in industry thinking. But extracting oil there comes with immense risks, thanks to the region’s remoteness and harsh conditions, a dearth of experience in offshore drilling there and a rudimentary understanding of the marine ecosystem. On the anniversary of the Deepwater Horizon oil spill, itself the result of oil and gas exploration under extreme conditions, we outline the disastrous consequences of an Arctic spill, and consider how to prevent it. Much is at stake. One area marked for development, the Chukchi Sea (see map), is among the world’s most productive ocean areas. Throughout the Arctic, algae support a food web that includes clams, crabs, fish and marine mammals; millions of seabirds migrate there and indigenous communities rely on a healthy marine ecosystem for their subsistence way of life. Warming conditions have expanded icefree areas, enabling more fishing, shipping and oil and gas extraction. Development is most advanced in Alaska, which is thought to hold by far the largest portion of Arctic offshore oil. Onshore exploration there during the 1960s led to discovery of the 25-billionbarrel Prudhoe Bay oil field in 1968. The first production in the Beaufort Sea began from a gravel island at the Endicott field in 1987. In the US Arctic, ocean leases worth more than US$7 billion have been sold since 1980, leading to approximately 35 exploration wells, the discovery of a 206-million-barrel field (Northstar) and continued industry interest.