Levels of the radioactive substance cesium-137, which fell into the sea due to Cold War-era atmospheric nuclear tests, have remained constant in waters near Japan for the past 10 years, study results showed Wednesday, reports the Kyodo News. Despite a decrease at the rate of cesium-137's half-life, or about 30 years, levels have remained constant due to the inflow of the substance on the oceanic current from the south. An analysis by the research team has shown that the substance went up to the stratosphere in nuclear explosions and fell in the largest quantities over the Pacific side of the Japanese archipelago and the East Coast of the U.S. after being carried by the jet stream and other high-altitude currents.