A major power outage across South Florida today brought normal life screeching to a halt, as traffic signals, schools, homes, and workplaces went dark around 1 p.m. At the peak of the outage, about 700,000 customers were left without power.

Throughout much of the afternoon, Florida Power & Light had a statement on the main page of its Web site saying it did not know the cause of the outage. But a company spokesperson assured the Miami Herald that the "failure is in no way a safety issue."

The problem reportedly began in an FPL substation in West Miami-Dade County and then spread through the transmission system, leading to an automatic shutdown of both nuclear reactors at FPL's Turkey Point power plant as well as three fossil-fuel-burning units. But that explanation raised questions for some, according to the paper:

Many experts ... remained puzzled. Normally, a problem at a single substation should not cause an outage of the magnitude that dominoed through the state Tuesday.

The Nuclear Regulatory Commission released a statement this afternoon saying the Turkey Point reactors shut down in response to an "undervoltage" caused when two power distribution lines failed following the substation malfunction. The reactor shutdown -- a safety measure to protect plant equipment from abnormal power line voltages -- is expected to last from 12 to 24 hours.

The NRC says it's monitoring the situation.