Big Bertha, the world’s largest tunnel boring machine, is clawing its way under Seattle to replace the aging above-ground Alaskan Way Viaduct. But in mid-December, something big blocked its path and stopped its forward progress under the city.  What is it? No one really knows, but one potential is it could be a glacial erratic. Terry Swanson from Earth and Space Sciences talks to KUOW about these glacial relics and their geologic history in the Puget Sound. Read more and listen to an interview on KUOW.