Foulden Maar is a fossil site near Middlemarch in Otago, New Zealand. The fossils were deposited in the small deep crater lake of a maar formed by a volcano in the Miocene era, around 23 million years ago. The crater lake existed for a period of around 130,000 years, and during this time it gradually filled up with diatomite, composed of annual layers of silica-shelled algae (diatoms). These layers of diatomite have preserved exceptional fossils of fish from the crater lake, and plants, spiders and insects from the sub-tropical forest that developed around the crater. The site is the only known maar of its kind in the Southern Hemisphere, and is one of New Zealand's pre-eminent fossil sites. A 2018 proposal to mine Foulden Maar for livestock-food additive has attracted significant public o