Kawainui Marsh in Kailua on Oʻahu is Hawaiʻi’s largest remaining wetland and the islands’ largest ancient freshwater fishpond. Once a shallow bay, it became a brackish lagoon as sea levels fell, and Polynesian settlers established communities near its springs, making it one of the earliest settled areas in Hawaiʻi. By about 1750, Hawaiians had developed a 400-acre freshwater fishpond with surrounding agriculture and nearby religious sites such as Ulupō Heiau. After European contact, population loss and later land changes led to decline and damaging new uses, but Kawainui remains a vital refuge for endangered Hawaiian water birds and a focus of ecological and cultural restoration.