Inside A 25,000-Year-Old Fire Ritual Hidden In An Australian Cave

Scientists Find Ancient Evidence of Deliberate Grass Burning in Hidden Cave

A limestone cave in southeastern Australia has preserved evidence of something far older and more purposeful than a campfire. Tucked into the hillside of East Gippsland, Cloggs Cave holds a record of human activity stretching back 25,000 years, roughly a thousand generations. A new study has zeroed in on microscopic clues locked inside the cave’s ancient soil layers: tiny silica particles left behind by plants, showing that Indigenous GunaiKurnai ancestors were carrying whole grass plants into the cave and burning them, again and again, across an extraordinary span of time.

Ancient peoples intentionally selected the grass, hauled it into a dimly lit limestone chamber, and burned it in thin, careful layers. Burned phytoliths recur throughout the 25,000-year sequence, showing that people repeatedly brought plants into the cave and burned them. Archaeological features and GunaiKurnai cultural knowledge suggest that at least some of these activities were ritual or spiritual, though the phytolith evidence alone cannot confirm that every episode served the same purpose. Published in Frontiers in Environmental Archaeology, the study was led by researchers at the Australian National University, Monash University, and GunaiKurnai Land and Waters Aboriginal Corporation.

Cloggs Cave has long fascinated archaeologists. Stone tools, a standing stone, miniature ritual fireplaces, and well-preserved plant material have all been unearthed across its layered sediments. This latest research focuses on phytoliths, microscopic silica structures that plants produce inside their own cells and that survive long after the plant itself has decayed. Because different parts of a grass plant produce different shapes of these silica bodies, researchers can identify not just that grass was present, but which parts of the plant were brought in.

Microscopic Silica Particles Show Grasses Dominated the Cave’s Record

Plants pull dissolved silica from the soil and deposit it inside their cells, creating rigid, microscopic shapes that remain behind long after the plant decays. Researchers analyzed sediment samples from two excavation areas, one spanning roughly 4,400 to 1,600 years ago, and another reaching back to about 25,000 years ago. Grass-specific silica particles dominated the record throughout.

Many of those particles had been blackened by fire, a change identifiable under a microscope. These burned particles were linked to thin, widespread ash layers largely free of charcoal, suggesting the fires primarily involved soft plant material. Researchers caution that wood may also have been used and simply left little surviving trace, since woody plants naturally produce far fewer phytoliths than grasses.

Burned Particles Point to People, Not Possums

A complication was the presence of Common Brushtail Possum droppings throughout the cave’s layers. Possums eat plants, and their droppings contain silica particles too. To sort out human from animal contributions, researchers analyzed individual droppings from the same layers. Unburned phytoliths in non-ash layers probably reflect a mixture of both possum and human activity, making that portion of the record harder to interpret with confidence. But not a single burned particle turned up in any possum dropping, and wind and water can’t explain the burned material either, since the cave’s narrow, curving entrance is sheltered from prevailing winds and flowing water has never reached its interior. Since fire blackens these particles, the burned fraction of the record points to people, not animals or weather, as the source.

Whole Grasses, Not Just Seeds

Perhaps the most telling finding concerns which parts of the grass plants were present in the ash-rich layers, where the evidence is clearest. Silica shapes from leaves, stems, and seed heads all turned up together in burned contexts. That means the burned material was not limited to isolated seeds or a single plant part. Whole grass plants, leaves, stems, and flowering portions alike, were carried inside and burned.

Combined with GunaiKurnai cultural knowledge and historical records, this points toward ceremonial or spiritual use as a strong interpretation for at least some of that burning, rather than ordinary fires kept only for warmth or cooking. Nineteenth-century accounts describe initiation ceremonies, the jeraeil, in which men were smeared with ash and charcoal and wore tufts of grass on their bodies. Two miniature fireplaces dating to around 11,000 and 12,000 years ago, each built with trimmed sticks coated in animal or human fat, closely match GunaiKurnai ritual installations described in that same ethnography.

Researchers were careful to note real limits, some plant identifications couldn’t be pinned down precisely, and higher phytolith counts in some layers can’t be read simply as more frequent visits, since different layers may represent unequal spans of time.

Even with those caveats, the convergence is hard to dismiss. Phytoliths, pollen, standing stones, ritual fireplaces, and GunaiKurnai oral history all point toward a secluded cave visited repeatedly across 25,000 years by people who carried grasses inside and burned them for reasons that, at least some of the time, went well beyond staying warm.

---