One of the crucial scenes in the novel A Passage to India takes place in the Caves of Malabar, in which every whisper comes back as thunder. Imagine my disappointment when I found out the caves were fictional... and my interest when it turns out they're based on a real phenomenon.

If you don't know it, E.M. Forster's A Passage to India is a novel that examines the social and political relationship between England and India in the 1920s. It's ightly acknowledged as one of the great English-language novels, and frequently appears on reading lists in high school and college literature courses. In the novel, the Malabar Caves are deep and complicated, and any noise made inside them, from the scrape of a match to the squeak of a boot heel, comes back as thunderous noise. The sound of the caves drives two British women temporarily mad. The caves are meant as a metaphor for both the coming revolution in India and the (at the time) unbreachable gap between the understanding of the Indians and the understanding of the British. While the Indians don't think much of the echoes, being used to even more impressive acoustics, the British can't handle them.

Later I found out that Forster based the Malabar Caves on the Balabar Caves, which have many beauties, but to my eternal disappointment, do not have the power to drive people into altered states of mind with sound. It turns out, though, there are caves around the world that do.

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One rock art expert believes that cave art is placed where it is as the result of specialized acoustics in caves. Clapping your hands in front of caves from Lascaux, France, to Chavín de Huántar, Peru, can create echoes that resemble all kinds of things. Researcher Steven Waller believes that the acoustic properties of the area around the paintings were meant to be as much a part of the art as the paintings themselves. Sometimes he and his colleagues can find art by clapping or shouting, and walking to the spot that has the most impressive echoes.

He believes art, sounds, and spirituality are bound up together. In the southwestern United States, paintings of thunderbirds tend to be in the areas where clapping or shouting returns the sound of thunder; meanwhile in France, horses and deer decorate spots on the wall where clapping will make it sound like a herd of hoofed animals running behind the cave walls. Acoustic graphs of a cave in Back Canyon, California, show that a single clap returns echoes of varying intensity — the sound can die away, and then come back stronger, only to die away again. And in the cave site in Peru, conch shell instruments were found, showing that people used the acoustics of the cave to create rituals. Waller and other researchers believe that people gathered around these spots, decorated with emblematic animals, and created thunder, or rain, or massive herds of prehistoric horses, during religious rituals.

Although the idea is not proved — or even provable — I do like it. It indicates there is a gulf between two mindsets, not separated by geographical distance, but by time. Two people standing in the same spot and experiencing the same phenomenon will perceive them completely differently. For one person, the experience is a hallucinatory moment, bringing together sound, vision, and religion, while the other will just see some pictures without taking note of anything else.

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