Complex sentence: Before that, seismologists believed Earth's core to be a single molten sphere, being unable, however, to explain careful measurements of seismic waves from earthquakes, which were inconsistent with the Earth having a single molten core.
Split into simpler sentences: Before that, scientists believed Earth's core to be a single molten sphere. However, seismologists had been unable to explain careful measurements of seismic waves from earthquakes, which were inconsistent with the Earth having a single molten core.