Pacific Gas & Electric Co. this week started using an untethered wireless robot to check "hard-to-reach" natural gas pipelines. The Explorer robot, which sends streaming video from inside the pipes to PG&E engineers, has magnetic sensors that can determine changes in a pipe's thickness. "It's going to become a key tool," said PG&E spokesman Jason King. "We've been searching across the country for cutting-edge technology to enhance the safety of our system."