Washington (CNN)The US military has detected "highly unusual and unprecedented levels" of North Korean submarine activity and evidence of an "ejection test" in the days following Pyongyang's second intercontinental ballistic missile launch this month, a defense official told CNN on Monday. An ejection test examines a missile's "cold-launch system," which uses high pressure steam to propel a missile out of the launch canister into the air before its engines ignite, preventing damage to the submarine or submersible barge that would launch the missile.