Moscow ? Two small Russian submarines completed a risky voyage Thursday deep below the North Pole, planting their country’s flag in a titanium capsule on the Arctic Ocean floor to symbolically claim what could be vast energy reserves beneath the seabed.

The subs dove some 2 1/2 miles to the Arctic shelf, where they collected geologic and water samples and dropped the canister. After spending most of the day below water, they surfaced near the pole, guided by four radio beacons on the perimeter of a football field-size hole cut in the ice.

Warming global temperatures have made the region increasingly open to shipping and energy exploration.