Something strange has been happening in Eastern Colorado at night.

Since the week of Christmas, giant drones measuring up to six feet across have been spotted in the sky at night, sometimes in swarms as large as 30. The Denver Post first reported these mysterious drone sightings in Northeast Colorado on December 23. Since then, sightings have spanned six counties across Colorado and Nebraska

Phillips County Sheriff Thomas Elliot had no answer for where the drones come from or who they belong to, but he has a rough grasp on their flying habits. “They’ve been doing a grid search, a grid pattern,” he told the Denver Post. “They fly one square and then they fly another square.”

The drones, estimated to have six-foot wingspans, have been flying over Phillips and Yuma counties every night for about the last week, Elliott said Monday. Each night, at least 17 drones appear at around 7:00 pm and disappear at around 10:00 pm, staying 200-300 feet in the air.

Read more: business insider