Pretty much wherever you are in the world, you can look up and see planes leaving behind long white trails. There's actually a scientific explanation for what they are - but not everyone believes it.
We're so used to seeing the trails now that most of us just accept them as another common feature of the sky, like clouds or birds.
When you actually think about what they are, you might lean more towards science. Or, if you're feeling a little more suspicious, you could opt to follow the more conspirational thread.
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This is one that's been growing in recent years, and suggests the white trails are made from chemicals being sprayed by the government.
The theory is one that's been around since 1996; the same year an Air Force research paper outlined a 'future weather modification system' which would use 'aerospace forces' to 'achieve military objectives'.
Some people appear to have drawn on the system mentioned in the paper to fuel their theory by believing the trails, known as 'chemtrails', are a way for the government to control the weather.
However, others have more sinister beliefs, suggesting the trails release toxic chemicals to poison humanity or try to control the minds of us down on Earth.
While it might seem pretty wild, there is some historical basis for the theory. In the 1950s and 1960s, the US released zinc cadmium sulfide into the air to test the dispersion of biological weapons.
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The chemical was believed to be nontoxic at the time, though it emerged that it could be cancerous through repeated exposure.
In 2021, Sijia Xiao, a PhD candidate at the University of California, looked into the chemtrails theory and found that believers 'pick and choose aspects that resonate with them, mixing in personal interpretations or selectively adopting parts of the theory'.
With social media only continuing to fuel the theory, experts have doubled down on the scientific explanation for the white lines and made clear there is no evidence for the existence of chemtrails.
Experts instead describe the lines as 'contrails', which is short for 'condensation trails'.
According to the University Corporation for Atmospheric Research, the trails appear when water vapour condenses and freezes around the exhaust from an aircraft.
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Harvard researchers looking in to the theory have also pointed out if people in power were using planes to release chemicals, a lot of people would have to be involved in the operation - making it much less likely that it would remain a secret.
Edward Snowden, who is known for leaking classified information from the National Security Agency, has also made clear that chemtrails are 'not a thing'.
Still, while the theory has been debunked, Xiao noted that the concerns which fuel the theory come from 'legitimate societal and environmental issues that warrant attention', such as distrust in the government or concerns about the environment.
Topics: Science, Conspiracy Theories, Environment, Health, Social Media, Weather