Astronomers say they have been stunned by the amount of energy released in a star explosion on the far side of our galaxy, 50,000 light-years away.
The position of SGR1806-20 in a radio image of the sky - 50,000 light-years away
The flash of radiation on 27 December was so powerful that it bounced off the Moon and lit up the Earth's atmosphere.
The blast occurred on the surface of an exotic kind of star - a super-magnetic neutron star called SGR 1806-20.
If the explosion had been within just 10 light-years, Earth could have suffered a mass extinction, it is said.
"We figure that it's probably the biggest explosion observed by humans within our galaxy since Johannes Kepler saw his supernova in 1604," Dr Rob Fender, of Southampton University, UK, told the BBC News website.
One calculation has the giant flare on SGR 1806-20 unleashing about 10,000 trillion trillion trillion watts.
"This is a once-in-a-lifetime event. We have observed an object only 20km across, on the other side of our galaxy, releasing more energy in a 10th of a second than the Sun emits in 100,000 years," said Dr Fender.
The event overwhelmed detectors on space-borne telescopes, such as the recently launched Swift observatory.
This facility was put above the Earth to detect and analyse gamma-ray bursts - very intense but fleeting flashes of radiation.
The giant flare it and other instruments caught in December has left scientists scrambling for superlatives.
Twenty institutes from around the world have joined the investigation and two teams are to report their findings in a forthcoming issue of the journal Nature.
Swift moved quickly to track down the source of the gamma-rays
The light detected from the giant flare was far brighter in gamma-rays than visible light or X-rays.
Research teams say the event can be traced to the magnetar SGR 1806-20.
This remarkable super-dense object is a neutron star - it is composed entirely of neutrons and is the remnant collapsed core of a once giant star.
Now, though, this remnant is just 20km across and spins so fast it completes one revolution every 7.5 seconds.
"It has this super-strong magnetic field and this produces some kind of structure which has undergone a rearrangement - it's an event that is sometimes characterised as a 'star-quake', a neutron star equivalent of an earthquake," explained Dr Fender.
"It's the only possible way we can think of releasing so much energy."
SGR 1806-20 is sited in the southern constellation Sagittarius. Its distance puts it beyond the centre of the Milky Way and a safe distance from Earth.
"Had this happened within 10 light-years of us, it would have severely damaged our atmosphere and would possibly have triggered a mass extinction," said Dr Bryan Gaensler, of the Harvard-Smithsonian Center for Astrophysics, who is the lead author on one of the forthcoming Nature papers.
"Fortunately there are no magnetars anywhere near us."
The initial burst of high-energy radiation subsided quickly but there continues to be an afterglow at longer radio wavelengths.
This radio emission persists as the shockwave from the explosion moves out through space, ploughing through nearby gas and exciting matter to extraordinary energies.
"We may go on observing this radio source for much of this year," Dr Fender said.
This work is being done at several centres around the globe, including at the UK's Multi-Element Radio-Linked Interferometer Network (Merlin) and the Joint Institute for VLBI (Very Long Baseline for Interferometry) in Europe - both large networks of linked radio telescopes.