Marine seismologists map the Earth's interior structure by looking at changes in the way sound travels through the planet's various layers. These sound patterns appear as "waves." Seismic energy travels as an elastic wave, meaning that it can be reflected from the seafloor ("reflection") or it can penetrate through the seafloor and the layers within the Earth's interior and be bent as it travels ("refraction"). An important tool used by marine seismologists to "read" patterns of sound waves is the seismogram. You have probably seen examples of seismograms, which depict measurements of earthquake patterns. The instrument that records these measurements is called a seismometer. Though it's hard to imagine, there are thousands of microearthquakes beneath the ocean floor each day! Although marine seismology has told us many things about the Earth's surface and interior over the last couple of decades, more work is needed to answer important questions such as: What do mid-ocean ridges, fracture zones, and continental margins look like in 3-D, and how did they form? What is the Earth's deep interior really like? What are structures in the upper mantle of the Earth like, and what is their role in plate interactions?