A think tank in Washington, D.C., has uncovered what it believes to be a heavily fortified, covert North Korean military base near the Chinese border that houses the nation's latest intercontinental ballistic missiles capable of reaching the U.S. mainland. The Center for Strategic and International Studies said in a report released Wednesday that it identified a secret base near the village of Sinpung-dong, approximately 17 miles from the Chinese border, The Wall Street Journal reported. The CSIS report stated it is one of approximately 20 ballistic missile bases, maintenance, support, missile storage, and warhead storage facilities which North Korea has…