The Boxing Day tsunami raised interest in tsunamis and how cataclysmic their effects can be. The SF Chronicle has an article about how soil cores can help determine when a tsunamic occurred in the past and give some sense as to its magnitude: Ancient sands reveal traces of huge tsunamis / Layers in soil cores could help predict future disasters: Typical evidence is a sheet of beach sand sandwiched between younger and older layers of ordinary soil, like a slice of ham between two slabs of sourdough.