The Aquitanian Sea (Helvoran: Ere Aquitaine, Spanish: Mar Cantábrico, French: Mer Gascogne), also known as the Sea of Biscay or the Cantabrian Sea, is a semi-inland sea east and south of The Kingdom of Helvore, west and south of France, and north of Spain. An island near the eastern coast of Helvore, called Erinier, forms the largest landmass in the Aquitanian Sea. A saline body of water, the Aquitanian Sea is sourced by the Atlantic Ocean through the Strait of Grenliviche (Helvoran: Chanson de Grenliviche, Spanish: Estrecho de Grenlivicho) in the south west, as well as numerous rivers throughout France, Helvore, and Spain.

The name 'Aquitanian' is believed to originate from the Latin name for the western French region of Gallia Aquitania, (modern-day 'Aquitaine' in France). The sea itself has often been used as a trading route between France, Helvore, and Spain, hence its nickname 'Sea of the Three Flags'.

Historically, the Aquitanian Sea was an important location in the Franco-Helvoran Wars and Napoleonic Wars, and there have been several conflicts over control of the island of Erinier. During World War II, the German invasion of Southern Helvore was based in the Aquitanian Sea.

Geographically, the Aquitanian Sea's average depth 1,744 metres (5,722 feet), and its deepest point is the Rift Abyss (Helvoran: Abysse de Voide, Spanish: Abismo Rift, French: Abîme Rift) at 4,735 metres.

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