A land of fire and ice
Heard Island is an external territory of Australia located in the Southern Ocean, approximately 4,000 kilometers southwest of Perth. The island is dominated by Big Ben, a massive active stratovolcano that is about 25 kilometers in diameter. Its highest point, Mawson Peak, rises to 2,745 meters (9,006 feet) above sea level, making it the highest mountain in any Australian state or territory outside of the Australian Antarctic Territory. The volcano has been active for about one million years, with eruptions recorded since 1881 and as recently as 2016.
Roughly 70 percent of Heard Island is covered by permanent snow and ice. The volcanic cone of Big Ben has 14 major glaciers that flow from the summit toward the sea. This permanent ice cover, combined with ongoing volcanic activity, creates a rare natural laboratory. Geothermal heat from the volcano melts the base of the glaciers, forming a network of subglacial caves and fumaroles. These ice caves are environments where liquid water, volcanic gases, and geothermally-warmed rock coexist, creating conditions found almost nowhere else on Earth.
Warm caves and hidden life
The interaction between volcanic steam and glacial ice creates unique, warm micro-environments. While outside temperatures are sub-Antarctic, temperatures inside these geothermally heated ice caves can be surprisingly mild. Similar volcanic cave systems on Mount Erebus in Antarctica have recorded temperatures as high as 25 degrees Celsius (77 degrees Fahrenheit).
These dark, moist environments on Heard Island are believed to have chemosynthetic ecosystems. These ecosystems are not dependent on sunlight for energy. Instead, microorganisms derive energy from chemical reactions, metabolizing sulfur and other compounds released in volcanic gases. While direct biological sampling of Heard Island's ice caves is extremely limited due to the island's remoteness and protected status, studies of analogous volcanic cave systems in Antarctica have revealed the DNA of diverse bacteria and fungi. Some DNA sequences discovered in these caves do not match any known species, suggesting the potential for undiscovered life forms uniquely adapted to this subglacial volcanic world. The conditions on Heard Island show how life might exist in extreme environments, both on Earth and potentially on other planets.