Submarine vents are also known, including a high solitary cone with two ridges off northeast Oʻahu, which is covered by pillow lavas and volcaniclastic sediments. Additional cones are found off the southwestern extension of the Koko Rift where they are situated on a southwestward extending ridge. Another set of submarine vents is found south of Diamond Head. It was once proposed that some seamounts (underwater mountains) off northeastern Oahu such as Tuscaloosa Seamount are related to the volcanic series; today however they are considered to be fragments of the giant Nuʻuanu Slide off northeastern Oʻahu.