A 190,000-year-old plumbing system
About 190,000 years ago, the Undara shield volcano erupted, unleashing a torrent of basaltic lava across the savanna of North Queensland. This single, massive eruption produced an estimated 23.3 cubic kilometers of molten rock, flowing at a temperature of around 1,200°C. The lava moved at a rate of about 1,000 cubic meters per second, following the paths of ancient, dry riverbeds. One of these flows traveled over 160 kilometers, making it one of the longest lava flows from a single volcano on Earth.
As this river of lava moved, its surface was exposed to the air and began to cool, forming a solid crust. Beneath this insulating roof, the molten interior kept flowing. When the eruption ceased and the last of the liquid rock drained away, it left behind a network of immense, hollow tunnels. Today, more than 70 of these caves have been found. Weaker sections of the tube roofs have collapsed over millennia, creating deep, moist depressions and providing access points into the subterranean world. Some individual caves, like Bayliss Cave, are over a kilometer long, with dimensions reaching 20 meters wide and 10 meters high. This entire system is preserved within Undara Volcanic National Park.
An isolated biological world
The dark, damp interiors of the lava tubes create stable micro-environments, different from the dry savanna woodland on the surface. These conditions support unique biological communities. The collapsed sections, known as dolines, allow light and moisture to enter, supporting pockets of semi-evergreen vine-thicket. These plant communities are relics, containing species with strong genetic links to the ancient supercontinent of Gondwana.
The caves themselves are biologically significant, with Bayliss Cave considered one of the most diverse caves in the world. They provide permanent shelter for large colonies of at least four insectivorous bat species, including the Eastern Horseshoe Bat (Rhinolophus megaphyllus) and the Eastern Bent-wing Bat (Miniopterus orianae oceanensis). These bat populations provide a food source for predators like pythons and owls, which hunt at the cave entrances. The tubes also harbor a diverse range of arthropods, with at least 78 species recorded. Many of these are troglobites—animals fully adapted to life in perpetual darkness—including specialized spiders, cockroaches, and planthoppers.