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Scientists believe that the biggest living organism on Earth is a fungus. But the heaviest organism, and the most massive organism, is a tree, or rather a giant colony of quaking aspen tree stems which has been growing across a hillside in the west of America for thousands of years. The colony - called Pando - was first discovered in the late 1960s. But it wasn't until many years later that scientists proved it was one genetic entity. Two of the scientists involved in researching Pando's story have been speaking to Louise Hidalgo about what they found out. Photo: Quaking aspen trees (Populus tremuloides) in autumn colours (Credit: Science Photo Library)