In certain conditions, tree sap may reach extreme negative pressures and emit popping sounds. Image credit: Jim Champion | http://bit.ly/11CPzC0 Rights information: http://bit.ly/c34Awz The crackling sounds wood makes as it burns are familiar to anyone who has roasted marshmallows before a campfire, but as trees get dry, they mysteriously crackle as well. Now scientists find these noises are apparently due to bubbles that loudly pop into existence within trees. These findings could lead to noninvasive means of testing tree health during droughts.
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