Russia's eerie "Doomsday Radio" whirred back into life for the first time in months on Monday - sparking wild speculation as a result.

The UVB-76 shortwave station, which typically transmits nothing but a monotonous buzz, came alive with two cryptic Russian voice messages.

Listeners picked up the codewords "NZHTI" and "HOTEL" along with a puzzling sequence of numbers: 38, 965, 78, 58, 88, and 37.

Some have suggested the numbers might be coordinates, while others heaped speculation that "something big" would happen on Monday night.

Vladimir Putin's eerie 'Doomsday Radio' whirred back into life for the first time in months on Monday

The mysterious radio station has been buzzing away since the 1970s in the height of the Cold War, earning the nickname "the Buzzer" as a result.

Its last transmission came on May 19 during a phone conversation between Donald Trump and Vladimir Putin.

Many believe the "Doomsday Radio" forms part of a covert military communications system, with some suggesting it could be linked to the "Dead Hand" automatic nuclear retaliation system.

The station, thought to be based outside Moscow, has used "NZHTI" as a call sign before, but the appearance of "HOTEL" alongside the numbers is a first.

PICTURED: The entrance to what's believed to be the UVB-76 shortwave station's home outside St Petersburg

Electronic and radio engineering professor David Stupples from City University of London has claimed Vladimir Putin's Government is "almost certainly" behind the transmissions.

Prof Stupples also issued a grim war warning, telling Popular Mechanics: "If it is the Russian Government, it wouldn't be for peaceful purposes."

While Dutch radio enthusiast Ary Boender, who runs Numbers Oddities, has tracked countless theories about the signal over the years.

"Some say that it is an old Soviet Dead Man's Switch that triggers a nuclear attack on the West when it stops buzzing," Mr Boender explained.

'Some say that it is an old Soviet Dead Man's Switch that triggers a nuclear attack on the West,' Ary Boender said

"Others say that it is a homing beacon for UFOs, or a mind control device with which the Russians can program your mind."

Most observers believe UVB-76 is a "numbers station", used to communicate encrypted messages between Russian spies or intelligence personnel.

Typically, these stations transmit numbers in groups of five, making it impossible for outsiders to pick up encoded words and sentences.

While one former high-ranking European official - and an expert on Soviet jamming of Western radio stations - known as "JM," maintains that the Buzzer's purpose is to transmit coded orders to military units within Russia.