What are sinks on an OS map?

On an OS (Ordnance Survey) map, sinks refer to low-lying areas where water collects, typically indicating features such as ponds, lakes, or marshes. These areas are often represented by blue shading or symbols that denote standing water. Sinks can also indicate depressions in the landscape that may not hold permanent water but are significant in the context of drainage and topography. Understanding these features is essential for various applications, including land use planning and environmental studies.