US, May 14 -- In the warm waters north of Papua New Guinea, where coral reefs plunge into some of the deepest nearshore canyons on Earth, a remarkable ocean world is still thriving. Spinner dolphins weave through the swells, gray reef sharks patrol reef drop-offs, and manta rays glide above coral gardens that scientists describe as among the healthiest left in the Pacific.

Now, Papua New Guinea (PNG) is taking a dramatic step to protect it.

At the inaugural Melanesian Ocean Summit in Port Moresby this week, the government announced plans to establish the Western Manus Marine Protected Area - a vast "no-take" sanctuary covering roughly 200,000 square kilometers of ocean, nearly the size of the United Kingdom. Once formally designated, it...