Building on a legacy left by President George W. Bush, President Obama has extended the reach of the Pacific Remote Islands National Marine Monument sixfold to nearly half a million square miles, turning it into the world's largest marine sanctuary, one fully protected from commercial fishing and deep-sea mining.

The move came via a presidential proclamation Thursday, issued under the 1906 Antiquities Act. The act allows a president to set aside for preservation structures or objects of historic or scientific interest on federal lands.

In 2009, former President Bush established the monument, centered on seven islands and atolls the US administers in the central Pacific Ocean. Each island or atoll was protected out to a distance of 50 nautical miles, giving the monument an area of 83,000 square miles.

In his proclamation, Mr. Obama extended the monument's reach out to the full 200-mile limit waters falling within US's exclusive economic zone around three of the islands and atolls. This raises the monument's collective area to 490,000 square miles.

Had the president included the other four islands in the expansion, the monument would have covered about 780,000 square miles. But after taking into account public comments, as well as the administration's own scientific assessment, the White House concluded that the expansion that became official today represents appropriately tailored and meaningful protections, according to administration officials.

Marine-conservation specialists are elated by the move.

"We're really thrilled. It's a huge step forward for the ocean. It's going to help spur other countries to take action," says Sarah Chasis, who heads the oceans program at the National Resources Defense Council in New York.

The White House signaled its intention to expand the marine monument in June during an international conference in Washington that focused on marine conservation. Secretary of State John Kerry convened the two-day meeting to build support for more-ambitious efforts to safeguard the environmental health of the world's oceans.

The key threats include over-fishing, pollution, and rising levels of carbon dioxide (CO2) from burning fossil fuels and from land-use changes. Rising CO2 levels have triggered global warming, and the oceans have become increasingly acidic as they take up much of the CO2 humans have pumped into the atmosphere.

The impact of warming on the oceans on marine life already is appearing as warm-water species migrate northward out of their historic ecological regions. Meanwhile, acidification threatens corals and many types of shell-building marine life, and by extension, the animals higher up the food chain that rely on them, researchers say.

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Obama extends vast marine reserve in central Pacific Ocean (+video)