The River of Ash That Raised a Reef: Queensland's Undara Lava Tubes

Seventy kilometres southwest of Mount Surprise, Queensland, a river of basalt once flowed through the earth, not over it. When the lava stopped moving, it left behind a network of tunnels—the longest and best-preserved lava tubes on the continent—and above them, a rainforest that has no business being in the dry savannah.

A River of Fire

About 190,000 years ago, the Undara Volcano erupted, releasing a flood of basalt that spread across more than 1,500 square kilometres of what is now northern Queensland. The lava was exceptionally fluid—a pāhoehoe flow, thin enough to travel at speeds of several kilometres per hour. As the outer surface cooled and crusted over, the molten interior stayed hot, carving a system of conduits that extended for more than 100 kilometres.

When the eruption ceased, the lava drained away, leaving hollow tubes behind. Some are 10 metres wide and 20 metres high. The longest runs for 22 kilometres—a single continuous tunnel, unbroken, its roof still intact after nearly two hundred millennia of tropical rain and seasonal fire.

The Roof That Became a Floor

Over the collapsed sections of these tubes, where the basalt roof has fallen in, something unexpected took hold. Rainwater, trapped in the depressions and shaded from the sun, created microclimates cool enough to sustain a relict rainforest. The dominant tree is the giant feather-leafed fern Angiopteris evecta, a species whose ancestors grew in the Carboniferous swamps 300 million years ago.

The ferns stand three metres tall, their fronds arching over the entrances to the lava tubes, where the temperature inside stays a steady 18 degrees Celsius year-round.

Beneath them, a community of vines, mosses, and orchids clings to the basalt walls. The contrast is jarring: just metres away, the surrounding country is open eucalypt woodland and spinifex, dry for most of the year. The boundary between the two worlds is sharp, drawn by the edge of the lava flow.

A Climate Archive in Stone

The Undara tubes are young in geological terms—the eruption that formed them happened well within the last glacial cycle, when humans had already been in Australia for tens of thousands of years. The basalt itself has barely weathered. Its dark, pitted surface still carries the ropey texture of the original flow.

This youth makes Undara useful for understanding how volcanic landscapes evolve. The tubes are still structurally sound, their collapse rates slow. But the weathering that has occurred—the formation of secondary minerals on the tube walls, the breakdown of basalt into clay in the collapse pits—gives a window into the early stages of soil formation in a monsoonal climate.

It also preserves a record of climate change. The relict rainforest that survives in the collapse craters is a fragment of a much wetter past. During the last interglacial, when rainfall was higher, closed forest covered much of the region. As the climate dried, the forest retreated to the few places where water lingered. The lava tubes, by providing shade and trapping moisture, became refuges.

What the Tunnels Hold

The tubes themselves are not empty. Bats roost in the deeper sections, depositing guano that accumulates in thick layers on the floor. Insects, spiders, and specialised cave-adapted invertebrates live in the dark zones, feeding on organic matter washed in from the surface. The system is a complete ecosystem, layered from the open sky to the black depths of the basalt.

Above ground, the collapse craters—known locally as "paddocks"—are scattered across the flow like sinkholes. Each is a self-contained garden, its flora distinct from its neighbours depending on how much light penetrates and how deep the soil has developed. Some hold dense thickets of fern and vine. Others are grassy clearings ringed by fig trees, their roots reaching down into the tubes below.

Undara is not the youngest lava field in Australia—that title belongs to the McBride Volcanic Province further north—but it is the most complete. The tubes are intact, the collapse craters are recent enough to still hold their rainforest, and the basalt flow has not yet been buried by sediment or erased by erosion. It is a landscape arrested in the moment just after the fire left, still cooling, still settling, still waiting for the next eruption to cover it again.