SEATTLE — Experts say they’ve at least partially solved the mystery of what’s been blocking a massive tunnel-boring machine beneath downtown Seattle for the past month: a steel pipe.The $80 million, five-story-tall machine, dubbed “Bertha,” is digging a new path for state Route 99, one of the region’s primary north-south arterials. But it became stuck Dec. 6, 60 feet below Seattle streets. In the ensuing weeks crews have probed, sifted and speculated in an effort to figure out what was causing the delay.Much of the city got in on the act. Some fingered a giant boulder. Students in a fifth-grade class wrote essays with their best guesses, including the remnants of a train left under the ruins when the city rebuilt after the Great Seattle Fire of 1889.Ivar’s restaurant, a local institution known for its publicity stunts, promoted the idea that it was a 70-foot “clamosaurus” that escaped the restaurant’s founder in 1937.The real answer is less exotic, and only a little less baffling: an 8-inch-diameter steel pipe that the state Department of Transportation already knew about.