A world without sunlight
In 1986, workers surveying land near Mangalia for a new power plant made an accidental discovery. Geologist Cristian Lascu descended an 18-meter-deep test shaft and entered a completely isolated environment, a cave that had been sealed off from the surface for an estimated 5.5 million years by thick, impermeable layers of clay and loess. This subterranean world, named Movile Cave, operates on principles entirely different from the surface. There is no sunlight. The food web is based on chemosynthesis.
The atmosphere inside is hostile to most surface organisms. Oxygen levels are between 7% and 10%, less than half of the 21% found in normal air. Conversely, carbon dioxide is 100 times more concentrated, and the air is thick with hydrogen sulfide and methane. This toxic gas mixture is produced by thermal, sulfurous waters seeping up from a deep aquifer. Bacteria and other microbes oxidize these chemicals to produce energy, forming thick, floating microbial mats on the water's surface and cave walls. These mats are the base of the entire ecosystem, feeding all the animals that live here in the constant 21°C (70°F) heat and 100% humidity.