For the second time in four months, scientists and government officials halted construction at Lawrence Livermore National Laboratory, site of the proposed National Ignition Facility. This time, crews trenching for electrical utilities at the site stumbled on the remains of a mammoth, a species extinct for at least 10,000 years. The hair-covered elephantlike mammals, marked by their long, curved tusks, once roamed the valleys surrounding the Livermore site. Mammoth bones are among the most commonly discovered fossils, according to one scientist at the facility. An unnamed paleontologist with a $30,000 contract has begun unearthing the mammoth's skull and jawbone. The work began Dec. 21 after the US Department of the Interior issued a permit under the Antiquities Act of 1906. The paleontologist is working cautiously to extract the skull and jawbone without damage. In September officials halted construction after they unearthed more than 100 electrical capacitors thought to contain hazardous materials. In January, construction workers at the site found the mammoth's shoulder bone and seven ribs 20 feet from the original find.