Gentoo penguin, Petermann Island, Antarctica (© Paul Souders/DanitaDelimont.com/Alamy)

Penguins thrive in some of Earth's coldest waters. Today's image shows a gentoo penguin beneath a vault of shimmering blue ice, its sharp silhouette etched against the frozen world around it. The gentoo is among the most agile swimmers of all penguins. With strong flippers and a sleek, tapered build, it can navigate icy waters with quick, confident bursts of speed. Its bright bill, bold white head stripe and upright posture make it instantly recognisable, even in colonies where thousands of birds gather along the shore.