An asteroid strike in the late 21st century that poisoned the Terra Nova colony with radiation, killed its adult settlers and left their children to grow up underground believing Earth had attacked them.
Terra Nova was Earth's first colony beyond the Solar system, reached by the ship Conestoga after a nine-year voyage. Relations with Earth soured when the colonists objected to plans for a second wave of settlers, and contact then ceased altogether.
The cause was an asteroid impact that spread a radioactive cloud over the settlement. The adults died, convinced that Earth had struck at them, while the young children survived in caves and became the Novans, a people with their own speech and a deep distrust of humans.
Enterprise NX-01 solves the mystery in 2151 in the episode Terra Nova. Archer's crew prove to the Novans that they are human and help them move to a safer part of the planet.