The fossils were deposited in the small deep crater lake of a maar formed around 23 million years ago by a volcano in the Miocene era.

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).

[4]: 20 A maar is a volcanic crater with a low rim that is formed in an explosion (known as a phreatomagmatic eruption), when magma or hot lava comes into contact with groundwater.

[7] The Foulden Maar lake formed in a volcanic crater during the Waitakian (early Miocene), approximately 23 million years ago.

These thin layers of silica are known as diatomite, as they are composed primarily of one diatom species, Encyonema jordaniforme Krammer, that grew on submerged rocks or aquatic plants in the lake.

[19] The fossil evidence derived from pollen and spores suggests a warm temperate or sub-tropical rain forest with canopy trees, with an understorey of shrubs, ferns and on the margins pioneer species.

[20][21] Leaf fossils from the deposit have been used to link past spikes in carbon dioxide levels with melting of Antarctic ice,[22][23][24] and the variations in the laminations of the diatomite have been studied to reveal the New Zealand climate from that time.

[1] Diatomite was extracted in small quantities from the site during World War II when transport difficulties hindered access to foreign sources.

[30] Plaman Resources' shareholders, the Iris Corporation and Burleigh Nominees, have allegedly been involved in corruption and humans-rights abuses and have unpaid debts.

[32] Plaman held the mining rights and planned to turn all of the fossil-containing diatomite into an additive for incorporating into the food of intensively farmed animals such as ducks and pigs.

[33] Plaman discussed with local councils applying to New Zealand's Provincial Growth Fund for help with the costs of building the processing plant, although no application was made.

[37] Previous mining at the site yielded low-quality diatomite only suitable for inclusion in concrete, rather than the high-quality product Plaman was marketing as "Black Pearl".

"[38] Animal nutrition experts expressed doubt that the product produced by Plaman will have any animal-health benefits, as there is no published data to support their claims.

[46] A wider group of people concerned about the loss of the unique fossil record described as "Dunedin's Pompeii" and an "irreplaceable treasure box"[35][47] launched a petition to preserve the site, garnering nearly 10,000 signatures in the first month.

[31][48][49] A leaked report by Goldman Sachs details the engagement of former Labour MP Clayton Cosgrove as a lobbyist to "secure approval" for the mine.

[53] Dunedin Mayor Dave Cull, who wrote a letter of support for the mining proposal, publicly called for clarification from Plaman after hearing details of the leaked Goldman Sachs report.

[54] Clutha Mayor Bryce Cadogan, who also supported the proposal, expressed frustration that commercial sensitivity prevented the release of all the information in favour of the mine.

[55][56] The council made this decision after hearing from Daphne Lee, a palaeontologist and associate professor at the University of Otago, explaining the scientific importance of the site.

[58] Sir Alan Mark, chair of the environmental group the Wise Response Society, called for the government to purchase the site and establish a geological reserve.

[62] As of September 2022, the Dunedin City Council had not proceeded with acquiring the land under the Public Works Act, and the protection of the site remained unclear.

[63] In February 2023, the Dunedin City Council reached an agreement with the receivers for Plaman Resources for the purchase 42 hectares (100 acres) of land at the site.