Mysterious Loud Jet Sounds Circling Over Austrian Mountaintop — But Nothing Visible in Sky.

“The roar was also like coming down in circles of about 1-2 km diameter, as if a huge airplane or jet was circling above our heads for 6 to 8 minutes (and then another 6 to 8 minutes with a 5 minute break in between), but compared to airplane noise we know, this was 100 times louder.”

— Richard, I. T. Architecture Software Designer, Vienna, Austria