Subduction zones are located where one of the Earth’s tectonic plates slides beneath another – the boundary between the tectonic plates is called the plate boundary fault. These plate boundary faults are capable of generating the largest earthquakes and tsunamis on Earth, such as the 2011 magnitude 9.0 earthquake in Japan. In the last two decades a completely new type of seismic phenomenon has been discovered at subduction zones: silent earthquakes or slow slip events. These ‘silent’ earthquakes happen over several weeks or even months and there can be no ground-shaking at all. In this lecture we discuss why this new discovery has turned the field of seismology upside down and how we study these strange earthquakes.