During military exercises on Monday, North Korea reportedly fired across the Northern Limit Line into South Korean waters. In response, South Korea fired K-9 self-propelled artillery into North Korean waters. Always up on the latest technology, North Korea apparently sent South Korea a fax early Monday morning informing the country of live-fire drills near the line. The back-and-forth comes just one day after North Korea declared on Sunday that it "would not rule out a new form of a nuclear test aimed at strengthening our nuclear deterrence. The U.S. had better ponder over this and stop acting rashly."