A mysterious flash has stunned the internet, after completely lighting up the skies above Kyiv late on Wednesday night.
It quickly and silently burnt itself out within just a few seconds before disappearing completely.
It’s easy to dismiss the strange display – which was seen around 10pm local time (7pm GMT) – as a new kind of attack from Russia, after more than a year of violent missile strikes launched across Ukraine.
An air raid siren also appeared to be set off just after the flash appeared – but there was no explosive noise alongside the light spectacle or any subsequent damage reported.
And the strange pattern of bright balls fluctuated in intensity, suggesting the Kremlin probably hasn’t developed some new high-tech weapon.
The Ukrainian Air Force is also pretty confident it wasn’t a Russian air attack.
So what was it?
Here are some theories...
1. Ukrainian air defences
Ukraine initially said it was their own air defences. After all, they received the US-made Patriot systems only hours before the flash – so it would make sense if they were testing their own brand new equipment.
But, Ukrainian officials then withdrew this claim, with the head of Kyiv’s military administration, Serhiy Popko explaining: “Air defence was not in operation.”
2. NASA activity
It wasn’t until 90 minutes after the flash that Ukrainian officials suggested early information implies the “phenomenon was the result of a NASA space satellite falling to Earth”.
That’s not too unusual and NASA is expecting a 300kg satellite (called RHESSI) to return to Earth this week – but that one is yet to drop, according to to the US space agency.
A NASA spokesman told Agence France-Presse (AFP): “That reentry has not yet occurred – RHESSI is still in orbit. NASA and the Department of Defence continue to track RHESSI. No other NASA satellite reentered the atmosphere earlier today.”
RHESSI was also nowhere near Ukraine at the time, according to Satellite-tracking website Satflare.
3. A meteorite
The main remaining explanation was that it was a meteorite, but by Thursday morning, officials in Ukraine still couldn’t be 100% sure.
Igor Korniyenko, deputy head of a control centre at Ukraine’s national space agency, eventually said: “We cannot identify what it was exactly, but our assumption is that it was a meteorite.
“Our observation devices showed it was a powerful explosion. We recorded it and determined where it took place.”
There is a chance that the flashes were triggered by ionised atmosphere heated up by the meteor’s impact. A similar event unfolded over Israel only recently.
Kyiv authorities just said: “What exactly it was is for experts to find out.”
4. Aliens
Of course, in between the official explanations, social media was flooded with speculation, conspiracy theories and memes.
The most popular one? Aliens.
Even the head of the Ukrainian president’s office, Andriy Yermak, tweeted an emoji of a UFO.