What was the Wow! signal, and why can't astronomers explain it?

It was a radio burst, caught on August 15, 1977 by Ohio State University's Big Ear telescope, that behaved the way a distant source on the sky would. A volunteer, Jerry Ehman, was going through data printed on line printer paper when he found a run of characters reading 6EQUJ5. He circled it and wrote "Wow!" beside it. Despite the popular idea, the string is not a coded message: it records the signal's intensity over time.

Why the excitement? The printout's highest value, "U", means an intensity thirty standard deviations above background noise. Big Ear swept the sky using Earth's rotation, so a steady source should be visible for 72 seconds, rising for the first 36 and falling after, and the Wow! signal showed these characteristics. Its frequency, close to the hydrogen line at 1420 megahertz, was the one Morrison and Cocconi had suggested in 1959 an extraterrestrial civilization might use.

The trouble is that nobody has ever seen it again. Follow-up searches, including with the more sensitive Very Large Array, found nothing, and the signal has never recurred. No explanation, terrestrial or otherwise, has been confirmed. As Douglas Vakoch has put it, the lack of replication means the Wow! signal has little credibility, while Ehman himself, in a 1997 paper, cautioned against "drawing vast conclusions from half-vast data". It is a strong signal with a missing second observation.