Only two-thirds of Millennials in the US are sure the world is round, according to a new survey.
Data and polling company YouGov surveyed more than 8000 people, asking: 'Do you believe the world is round or flat?'
Overall, 84 percent said they have always known the Earth is round. Fourteen percent said they weren't sure, and only 2 percent said it was definitely flat.
But the younger the age group, the more likely people were to doubt the world's rounded shape.
While 94 percent of over-55s said it was definitely round, only 66 percent of 18- to 24-year-olds did.
Of that the youngest group, 30 percent said they weren't sure and 4 percent were convinced the Earth is flat.
The Earth is not flat, it's an oblate spheroid - roughly spherical, but slightly flattened.
The researchers said there was a strong correlation between believing in a flat Earth and being "very religious".
Celebrity astrophysicist Neil deGrasse Tyson last month took to YouTube - where many of the flat Earth conspiracy theorists hang out - to explain why there is no doubt whatsoever the Earth is round.