The ocean floor is flat therefore the Earth is flat

Abyssal plains: Flat seafloor area at an abyssal depth (3,000 to 6,000 m [10,000 to 20,000 feet]), generally adjacent to a continent. These submarine surfaces vary in depth only from 10 to 100 cm per kilometre of horizontal distance. Irregular in outline but generally elongate along continental margins, the larger plains are hundreds of kilometres wide and thousands of kilometres long. In the North Atlantic the Sohm Plain alone has an area of approximately 900,000 square km (350,000 square miles of flatness).