A massive power blackout hit southern Brazil Jan. 21, including the metropolitan areas of Sao Paulo and Rio de Janeiro, stranding thousands in subways for almost two hours. The cause of the power failure, the worst in three years, was said to be a broken transmission cable rather than the energy shortage that forced power rationing throughout the country last June. Officials reported that the transmission problem launched a series of automatic preventive shutdowns but the grid was back to normal later in the day. Air conditioning was off in the sweltering heat and traffic was snarled everywhere with no traffic lights functioning. If the reports are accurate, there should be no additional outages because reservoirs that fuel the power supply are rising to normal levels.