In the Middle Miocene, 15–20 million years ago, the warm, shallow Pannonian Sea covered the region. Avala, Fruška Gora, and Vršačke Mountains emerged as islands. Corals, red algae, foraminifera, and bryozoans built a massive reef at Tašmajdan. When the sea receded, a solid limestone massif remained, which humans later quarried to build Belgrade.