COMMACK - LIPA announced today that the explosion that rocked Commack Saturday was caused by an underground transformer that became overloaded. It says underground transformers are very safe and have a distinctive look. Typically, they have a large round manhole-like cover and are usually located on private properties near the curb. "Most of the time, those things will fail internally and shut themselves off," says Nick Lizanich, of LIPA. "In this case, it failed violently." According to LIPA, all of the transformers, including the one on Peconic Court that blew up over the weekend, are inspected routinely. The utility adds that underground explosions are extremely rare.Still, in the aftermath of the blast, residents are asking LIPA to clearly mark the locations of its underground transformers.