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Drill a hole into the heart of a large tree and insert a microphone. Mount the amplifier and speaker in an empty room and adjust the volume to make audible any sound that might come from the tree.
Drill a hole about a mile into the earth and drop a microphone to within a few feet of the bottom. Mount the amplifier and speaker in a very large empty room and adjust the volume to make audible any sounds that might come from the cavity.
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