According to a new study from the University of Oslo, one side of the Earth’s interior is losing heat much faster than the other—and the culprit is nearly as old as time.

The study, published in Geophysical Research Letters, calculates how “insulated” each hemisphere was by continental mass during the last 400 million years. Insulation is a key quality that keeps heat inside rather than releasing it. The pattern goes all the way back to Pangaea.

The interior of Earth is a red hot liquid that warms the entire world from the inside. It spins as well, producing gravity as well as the Earth’s magnetic field. This keeps our protective atmosphere close to the surface of the Earth.

This interior will continue to cool in the very long term, until Earth approaches Mars. The surprise in the new study is how unevenly the heat is dissipating, but the reason makes intuitive sense: Parts of Earth have been insulated by more landmass, creating something of a Thermos layer that traps heat.

This is in contrast to how the Earth loses the majority of its heat: “Earth’s thermal evolution is largely controlled by the rate of heat loss through the oceanic lithosphere,” the study authors write. Why is this the site of the most tragedy? We need a quick-and-dirty run-through of continental drift for that.

The Earth’s mantle works similar to a convection oven that powers a treadmill. Every day, the seafloor surface shifts somewhat; new seafloor is formed by magma erupting at the continental divide, while old seafloor gets smashed and melts beneath existing continental landmasses.

The scientists created a model that splits Earth into African and Pacific hemispheres, then divides Earth’s entire surface into a grid by half degrees of latitude and longitude.

Several previous models for variables like seafloor age and continental placements during the last 400 million years were integrated by the scientists. The team then calculated how much heat each grid cell contains over its lifetime. This provided the path for the researchers to determine the entire rate of cooling, and they discovered that the Pacific side cooled far faster.

Mantle heat loss (oceanic + continental) over the last 400 million years. Blue and orange lines depict regions above the Pacific and African major low shear velocity provinces. The Pacific and African hemispheres are divided by dashed, light-colored meridians. KARLSEN, ET. AL./GEOPHYSICAL RESEARCH LETTERS

The seafloor is far thinner than the bulky landmass, and temperature from within Earth is “quenched” by the enormous volume of cold water that’s above it. Think of the gigantic Pacific Ocean compared with the opposite-side landmasses of Africa, Europe, and Asia—it makes sense that heat dissipates more quickly from the biggest seafloor in the world.

Previous study on this seabed effect only went back 230 million years, so the new model, which goes back 400 million years, nearly doubles the timeframe under investigation.

The findings contain an unexpected contradiction. The Pacific hemisphere has cooled about 50 Kelvin more than the African hemisphere, but the “consistently higher plate velocities of the Pacific hemisphere during the past 400 [million years]” suggest the Pacific was much hotter at a certain moment in time.

Was it covered by landmass in the distant past, trapping greater heat inside? Other theories exist, but today’s considerable tectonic activity in the Pacific indicates a heat discrepancy. The melting of the mantle allows the plates to slide and bang together.