Holtsós is a tidal lagoon located in south Iceland, near Eyjafjallajökull. It's about 4 kilometers long and 2 kilometers wide, with a maximum depth of 2 meters. The lagoon is fed by the Holtsá river and is separated from the Atlantic Ocean by a narrow strip of sand. It's a popular spot for Eurasian oystercatchers and other species like Gammarus zaddachi and Gammarus duebeni. The area has a rich history, with mentions in medieval texts and Icelandic folktales. Holtsós is known for its occasional flooding, especially when the river levels rise.