Proof of the existence of the passage in the mid-1800s only revealed how difficult its transit would be, and it was not until the early 20th cent. that the first transit was accomplished. The first commercial ship to successfully transit the Northwest Passage was the SS Manhattan, an ice-breaking tanker, in 1969. In 1988 the United States, which contends that the passage is an international strait through which all navigation may pass unimpeded, and Canada, in whose sovereign waters that passage lies, agreed that U.S. icebreakers could cross its arctic waters, but only after approval on a case-by-case basis. Although the straits that form the passage have historically been ice-clogged year-round, by 2007 warming in the Arctic made the area almost ice-free in late summer.
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