The Tweed Volcano was a shield volcano, the gently sloping kind built from fluid lava spreading wide rather than blasting upward. Its eruptions, recorded in the rocks as the Lamington Volcanics, played out in layers over roughly three million years. First came floods of dark basalt, then a phase of pale, glassy rhyolite, then basalt again - each lava with its own chemistry, stacked like the pages of a geological book. At its peak the dome rose over two kilometres above the sea, a giant on the early-Miocene coast. The volcano fell silent as the continent carried it away from the hotspot that fed it, and what had been an engine of fire became a monument waiting to be carved.

A true caldera collapses when a volcano empties itself. This one was sculpted instead - excavated grain by grain over 20 million years by the rivers and rain that radiate out from the old summit. The result is an erosion caldera widely cited as the largest in the Southern Hemisphere: a basin more than 40 kilometres across and over a thousand metres deep, on a scale that rivals Africa's famous Ngorongoro Crater. Its rim is a near-continuous wall of ranges and plateaus - the Tweed Range, the Lamington and Springbrook plateaus, the Nightcap Range - cradling the valley on almost every side. Hard volcanic rock resisted; softer material washed to the sea; and the slow arithmetic of weather hollowed out a bowl big enough to hold a whole region.

What the volcano destroyed in height, it repaid in fertility. The caldera floor is carpeted in deep volcanic soils that today grow some of Australia's most productive farmland, with sugarcane the signature crop spreading green across the valley. The ranges that ring it catch the moist coastal air and wring it into rain, feeding the ancient subtropical rainforests that survive on the slopes - the Gondwana Rainforests, a World Heritage treasure of living fossils. Lamington National Park alone, draped along the northern rim, is laced with more than 500 waterfalls and cascades. Cliffs and gorges host plants found almost nowhere else, clinging to bare rock high above the valley. The locals have a name for this whole vast amphitheatre: the Green Cauldron.

Erosion did more than carve scenery - it exposed the volcano's hidden anatomy. Stripping away the outer dome revealed the central feeders that once fed the eruptions, now standing as isolated peaks. The most striking is Wollumbin / Mount Warning, the solidified plug at the very heart of the system, but it has company - Mount Nullum and a scatter of old vents reaching as far as Nimbin. Together these survivors form what geologists call the Mount Warning Central Complex, a core of unusual rocks - syenite, granite, dolerite - that cooled deep inside the volcano and only saw daylight after the mountain above them was gone. To trace the Tweed Valley today is to walk through the cross-section of a volcano, opened by 23 million years of patient ruin.