Scientists have discovered two continent-sized regions hidden deep within Earth’s mantle, referred to in a recent study as “islands.” Research from Utrecht University reveals that these regions are not only hotter than their surroundings, but are also ancient, dating back at least half a billion years or longer.

The findings challenge conventional theories about the Earth’s mantle being a well-mixed and rapidly flowing system. Instead, the research suggests there is much less flow within the mantle than previously believed.