Scientists have discovered the Earth's inner core may be far hotter than previously thought, putting it at 6000°C - as hot as the Sun's surface.
Researchers at the European Synchrotron Radiation Facility in France have simulated the extreme conditions found at the centre of the earth, the BBC reports.
The solid iron core is actually crystalline, surrounded by liquid, and the temperature at which that crystal can form had been a subject of long-running debate.
Experiments outlined in Science used X-rays to probe tiny samples of iron at extraordinary pressures to examine how the iron crystals form and melt.
Measurements in the early 1990s of iron's "melting curves" from which the core's temperature can be deduced suggested a core temperature of about 5000°C.
The team revisited those 20-year-old measurements, making use of the European Synchrotron Radiation Facility - one of the world's most intense sources of X-rays.
To replicate the enormous pressures at the core boundary - more than a million times the pressure at sea level - they used a device called a diamond anvil cell - essentially a tiny sample held between the points of two precision-machined synthetic diamonds.
The iron samples were subjected to high pressures and high temperatures using a laser. Scientists then bounced X-rays off the nuclei of the iron atoms and watched how the pattern changed as the iron changed from solid to liquid.
Those diffraction patterns give more insight into partially molten states of iron, which the team believes were what the researchers were measuring in the first experiments.
They suggest a core temperature of about 6,000°C, give or take 500°C - roughly that of the Sun's surface.