What gave U-Boats open access to the Atlantic?

U-Boats gained open access to the Atlantic primarily due to the lack of effective Allied naval patrols and the limitations of anti-submarine warfare early in World War I and World War II. The vastness of the ocean, combined with the initial surprise and technological advantages of German submarine warfare, allowed U-Boats to exploit shipping routes with relative impunity. Additionally, the use of strategies like unrestricted submarine warfare further facilitated their operations, leading to significant disruptions in Allied supply lines.