The 121-hectare national park situated in Victoria's Keilor Plains is named after its main attraction: straight basalt columns that resemble 20-meter-high organ pipes. The park is undergoing a gradual restoration process to bring it back to its natural state and is the nearest national park to Melbourne's city centre. The Organ Pipes were formed over a million years ago when a lava flow, around 70 meters thick, came from Mt Holden and other volcanoes near Sunbury. As the lava cooled slowly and evenly in deep layers, it contracted, causing vertical surface cracks to develop. Over time, the cracks lengthened and eventually divided into clusters of hexagonal or irregular pentagonal columns, creating the remarkable Organ Pipes.