For 17 years, the Parkes radio telescope in Australia has been receiving strange, intermittent signals dubbed "perytons". A PhD student finally figured out where they were coming from: Nearby microwaves.
For years, the "perytons" had been a confusing riddle. They had a frequency signature extremely similar to "fast radio bursts", signals that come from outside the galaxy, possibly produced by neutron stars getting sucked into black holes. In contrast, the perytons weren't coming from outer space. They were coming from something on Earth; the scientists could figure that much out. But what force on the planet was producing them? Lightning? Cosmic rays hitting the atmosphere?
PhD student Emily Petroff did some detective work and finally located the real source: A couple of 27-year-old microwave ovens that the scientists used to heat up their meals:
It turns out that event was caused by impatient astronomers and other staff opening microwave oven doors before the timer ended. While the ovens operate at the higher frequency during normal operation, opening the doors early causes the oven's magnetrons to issue a very brief burst at the lower 1.4GHz frequency as they power down.
The researchers concluded: "Radio emission escaping from microwave ovens during the magnetron shut-down phase neatly explain all of the observed properties of the peryton signals."
If you want to read Petroff's white paper describing her results, it's here, and is a pretty fun read.
(That CC-2.0-licensed cat/microwave pic via the Flickr stream of David Shane!)