Which dark albedo feature on Mars was once thought to be a sea?

The dark albedo feature on Mars once thought to be a sea was Syrtis Major Planum.

Syrtis Major is one of Mars’s most prominent dark surface markings. Dutch astronomer Christiaan Huygens recorded it in 1659 and used its apparent movement to estimate the Martian day. Later mapmakers gave it imaginative names including the Hourglass Sea, reflecting the belief that the dark region might be a shallow body of water.

The feature’s appearance changed with the Martian seasons, encouraging 19th-century ideas about water and even vegetation. Giovanni Schiaparelli popularized the name Syrtis Major after observations during Mars’s close approach in 1877. The name comes from the ancient Roman name for the Gulf of Sidra on Earth.

Spacecraft observations eventually showed that Syrtis Major is a broad volcanic plain, not a sea. Its dark tone is largely produced by basaltic terrain and the shifting distribution of windblown dust and sand. It should not be confused with Arabia Terra or the bright polar ice caps.