US President George W Bush is to announce the creation of the world's largest marine protection area spanning some 505,000 square kilometres in the Pacific Ocean, a spokesman said.
The three areas to be designated as "marine national monuments" include the Mariana Trench and northern Mariana Islands, the Rose Atoll located in American Samoa, and a chain of remote islands in the central Pacific.
Establishing marine national monuments aims to ensure that certain resources are protected, such as rare fish and bird species, coral reefs and underwater active volcanoes, a top Bush aide on the environment said.
"This is very, very big," James Connaughton, chairman of the White House Council on Environmental Quality Issues, told reporters.
"In the last several years, it's on par with what we've been able to accomplish on land over the course of the last 100 years," he said.
The total area will "comprise the largest areas of ocean or ocean seabed set aside as marine protected areas in the world."
Collectively, the three areas will nudge out the Phoenix Island Protected Area, established in 2008 by the South Pacific nation of Kiribati as the world's largest protected area.
They also top Mr Bush's last such announcement of a marine protection area in 2006 - the 363,000 square kilometres of Pacific Ocean near the north-western Hawaiian islands.
Among the protected areas are the 2,400-kilometre Mariana Trench, including submerged active volcanoes and hydrothermal vents that run along the Mariana Island chain, an area that contains the deepest point on Earth.
The Pacific Remote Islands National Monument will comprise areas with coral reef ecosystems that are home to sharks, endangered turtles and millions of seabirds off seven areas: Kingman Reef; Palmyra Atoll; Howland, Baker and Jarvis Islands; Johnston Atoll; and Wake Island.
Finally, the Rose Atoll Marine National Monument will be named around a small but dense coral reef known for its pink shade at Rose Atoll, a remote area around American Samoa.
For scientists, the designations are "wonderful opportunities. You don't get a better natural laboratory than we have in these places," said Roger McManus, vice president for global marine programs at the environmental group Conservation International.
"What we have in these islands is a natural laboratory to understand how humans affect coral reefs," said McManus.
Naming them as marine monuments "will do a lot to protect the coral reefs and also do a lot restore fish populations in the regions," Mr McManus said.