A Cathedral of Calcite
Beneath the Causse Méjean, a limestone plateau in southern France, lies a single, immense chamber called the Aven Armand. The main hall, or "Grand Salle," measures approximately 100 meters long by 55 meters wide. Its total volume is estimated at 200,000 cubic meters, large enough to contain the Notre-Dame Cathedral in Paris. This enormous void was formed by the slow dissolution of Jurassic limestone by water over millions of years, leading to the eventual collapse of the cave's ceiling.
The cave's existence was unknown until September 19, 1897, when it was first explored by Louis Armand, a local blacksmith, and Édouard-Alfred Martel, a pioneer of modern speleology. Their initial descent was a hazardous 75-meter drop through a narrow vertical shaft, or "aven," in the ceiling. Today, visitors enter through a 208-meter-long tunnel excavated in 1926 and descend via a funicular railway, first installed in 1963. This funicular is distinctive for its use of rubber tires on a concrete track, a design intended to minimize vibrations that could damage the cave's delicate formations.
The Virgin Forest
The floor of the Aven Armand is covered by a dense collection of over 400 stalagmites, collectively known as the "Forêt Vierge" or Virgin Forest. These structures are formed by the precipitation of calcium carbonate from water dripping from the cave's ceiling. The forest contains a variety of shapes, including thin "draperies" and wide "flowstones," with many individual stalagmites reaching heights of 15 to 20 meters.
The most prominent feature is the Great Stalagmite (Le Grand Stalagmite), which stands 30 meters tall, making it the tallest known stalagmite in the world. The growth of these formations is exceptionally slow, dictated by the drip rate and mineral content of the water. The constant internal temperature of the cave provides a stable environment for this continuous depositional process. This subterranean forest presents a detailed geological record of the water that has percolated through the limestone above for thousands of years. The cave was opened to the public on June 11, 1927, thirty years after its discovery.