After leaving Nootka Sound, Cook explored and mapped the coast all the way to the Bering Strait, on the way identifying what came to be known as Cook Inlet in Alaska. In a single visit, Cook charted the majority of the North American north-west coastline on world maps for the first time, determined the extent of Alaska, and closed the gaps in Russian (from the West) and Spanish (from the South) exploratory probes of the Northern limits of the Pacific.
By the second week of August 1778 Cook was through the Bering Strait, sailing into the Chukchi Sea. He headed north-east up the coast of Alaska until he was blocked by sea ice. His furthest north was 70 degrees 44 minutes. Cook then sailed west to the Siberian coast, and then south-east down the Siberian coast back to the Bering Strait. By early September 1778 he was back in the Bering Sea to begin the trip to the Sandwich (Hawaiian) Islands. He became increasingly frustrated on this voyage, and perhaps began to suffer from a stomach ailment; it has been speculated that this led to irrational behaviour towards his crew, such as forcing them to eat walrus meat, which they had pronounced inedible.