Picture this, you're a scientist at the National Weather Service, and you see the image above – a 110 kilometre (70 mile) blob flying across Colorado – on your weather radar.
Is it birds? Aliens after some Colorado craft beer?
"Look at what's flying into Denver!" tweeted the NWS Boulder office. "Radar from last hour showing what we believe to be birds. Any bird experts know what kind?"
Look at what's flying into Denver! Radar from last hour showing what we believe to be birds. Any bird experts know what kind? #ornithology pic.twitter.com/EAqzdMwpFU— NWS Boulder (@NWSBoulder) October 3, 2017
"We hadn't seen a signature like that in a while," said NWS meteorologist Paul Schlatter, who first spotted the radar blip, to CBS News.
"We detect migrating birds all the time, but they were flying north to south."
This direction of travel would be very unusual for migratory birds for the time of year.
Turns out they weren't birds at all, as eyewitness accounts soon confirmed. Instead, this crazy mass was actually migrating painted lady butterflies.
1/4: We believe migrating butterflies are the cause of yesterdays radar signature. Thanks for all the reports and sightings! #cowx— NWS Boulder (@NWSBoulder) October 4, 2017
2/4: Insects rarely produce such a coherent radar signature. Migrating birds do all the time. #cowx— NWS Boulder (@NWSBoulder) October 4, 2017
This is what they look like down on the ground:
Butterflies 🦋 are majestic. Enjoying nature on my way to coaching a client. #businesscoach pic.twitter.com/NkT12SrBXF— Elizabeth Suárez, MBA, ADR 🇵🇷🇺🇸🗣 (@elizabethsuarez) October 5, 2017
Amazing! The Painted Lady butterfly migration thru Denver @botanic #butterflies #denver pic.twitter.com/MqQ2BE0sEI— KiKi Cannon (@kikidenver) October 5, 2017
But here's the thing – butterflies don't usually show up on weather radars. So what's different this time?
3/4 Things with big wings need to fly together in the same direction with the wind to generate that signature in ZDR (purple image). #cowx— NWS Boulder (@NWSBoulder) October 4, 2017
4/4: Migrating butterflies in high quantities explains it. Today, the butterflies are staying close to the ground. #cowx #Science pic.twitter.com/rkpwmPRnsi— NWS Boulder (@NWSBoulder) October 4, 2017
Giant migrations, and good conditions, lead to the insects showing up on radar – and apparently the people in Colorado have been pulling out all the stops to make the state butterfly friendly.
Mary Ann Colley with the Butterfly Pavilion in Westminster, Colorado, said there are a number of factors contributing to the large numbers.
"We've been planting the right types of plants, and with the fantastic weather we've been having, it's the right combination of success for these butterflies," she told KUSA.
Seriously, nature is amazing.