The National Security Agency has warned that aliens could contact humans through high-frequency signals, as experts say "we're not alone in the universe."
A declassified document from the NSA titled "Communication with Extraterrestrial Intelligence" explains the various methods aliens could use to communicate.
One way extraterrestrial civilizations could reach out to humanity is through a signal with a frequency of 1420.405752 megacycles per second and a wavelength of 8.26 inches, according to the NSA.
Atomic hydrogen "permeates space in great clouds," and has this radiation frequency, which can pass through the Earth's atmosphere.
Advanced civilizations familiar with radio frequencies would likely understand this form of communication.
"We are not alone in the universe," reads the NSA document. "A few years ago, this notion seemed farfetched; today, the existence of extraterrestrial intelligence is taken for granted by most scientists."
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