Geologists announced that the search was over. In Canada, just outside Toronto, they found a lake. The Wilford Lake is a collapsed limestone cave. Each year, as its waters get warmer, chemicals left over from the collapse react to form white crystals. There's pollen trapped between layers from the 13th century and soot from European colonizers in the 19th century. But most importantly, for the geologists, there's a huge uptick in plutonium just after 1945. This spike in plutonium is what's being proposed as the primary marker for the new geological epoch.