Astronomers at the Parkes Observatory in New South Wales, Australia, had been noticing strange signals coming through the radio telescope since 1998. The signals were identified as perytons. They were observed several times each year. Lightning was thought to be a possible cause.
Eventually they looked at the facility’s microwave oven. At first no perytons were detected. Then they opened the microwave’s door a few seconds before the microwaving had ended. They found that opening the door like that causes the interference.
These findings have been published in the notices of the Royal Astronomical Society.