The researchers used SS precursor analysis to study Earth’s mantle transition zone, analyzing seismic waves from over 34,000 recordings. They found the transition zone east of the East Pacific Rise is 12 miles thicker due to a depression of the 660-km discontinuity, indicating colder material from an ancient subducted plate stuck for over 100 million years. Limitations include uncertain timing of the subduction event and potential mantle composition changes. The findings offer insights into Pacific plate tectonics and suggest long-lasting effects of subducted plates on mantle structure.