The strange moving lights were seen whizzing across the morning skies over the Blades' ground earlier today.
The out-of-this-world CCTV footage shows a number of white blobs moving quickly above the famous ground - but zipping around the clouds in haphazard directions.
The clip was shot at around 7am this morning by a camera installed at the premises of Vision CCTV, a security firm based on Hill Street.
Spokesman Tim Ralston said: "The camera was filming from the top floor of the new flats in our car park.
"We've never seen anything like this before. We all looked it and realised they were some kind of UFOs.
"You will notice that there is an aeroplane light but these things are moving very fast and in weird directions. We're not really sure what it is we've got on camera."
* What do you think? Do you have an explanation? Let us know.