This is the sound of a plant dying of thirst. Heartbreaking isn't it?
As a plant's water source dries out, small bubbles form in the xylem - the hollow strands that carry water from the soil to the leaves of vascular plants.
The recording was made 30 years ago by Dr. Kim Ritman, using a very low-fi phone receiver with a pin soldered onto it to amplify the sound.
Ritman, who is now chief scientist at Australia's Department of Agriculture, spent a good part of his PhD poking the pin into leaf stems of plants and recording the clicks as bubbles formed. The idea was to see if the diameter of the xylem determined the frequency of the sound, and he found that the larger the xylem, the lower the clicking sound.
"In general, our hypothesis that larger conduits produced lower frequency signals and smaller units at the ultrasonic frequencies was supported," he writes in his study Acoustic Emissions from Plants: Ultrasonic and Audible Compared.
This story originally appeared on ABC Science.