Heating a substance causes molecules to speed up and spread slightly further apart, occupying a larger volume that results in a decrease in density. Hot water is less dense and will float on room-temperature water.

Temperature and pressure progressively increase with increased proximity to Earth’s core. Recent studies indicate the core’s temperature may be close to 11,000 degrees Fahrenheit; that’s nearly 2,000 degrees warmer than previously thought and hotter than the surface of the Sun, according to a 2013 Forbes article.

The Earth gets hotter as one travels towards the core, known as the geothermal gradient. The geothermal gradient is the amount that the Earth’s temperature increases with depth. On average, the temperature increases by about 25°C for every kilometer of depth.

The atmosphere and Earth’s interior are layered by density. Gravity pulls more strongly on denser materials so denser materials are at the center of things. Earth’s core, at its center, is denser than its crust. This protects life on Earth from dangerous solar particles that head toward Earth.

The density of an object is one of its most important and easily-measured physical properties. Densities are widely used to identify pure substances and to characterize and estimate the composition of many kinds of mixtures.

In new research, scientists studying what the conditions at the core should be like found that the center of the Earth is way hotter than we thought—around 1,800 degrees hotter, putting the temperature at a staggering 10,800 degrees Fahrenheit. The apparatus produces massive pressures and superheated temperatures.

There are three main sources of heat in the deep earth: (1) heat from when the planet formed and accreted, which has not yet been lost; (2) frictional heating, caused by denser core material sinking to the center of the planet; and (3) heat from the decay of radioactive elements.