Using advanced computer modeling, scientists from the California Institute of Technology and Shanghai Astronomical Observatory determined that pieces of a protoplanet the size of Mars are buried below the Earth’s crust. The debris is believed to be a result of a collision between the protoplanet and early Earth about 4.5 billion years ago, which also formed our moon. Because the composition of the protoplanet contained so much dense iron, the scientists believe that most of the shards sunk toward the Earth’s core and that concentrations of this alien debris beneath the African and Pacific tectonic plates explain the mystery of unusual seismic activity in those areas.