NEW: Underwater IED detonated after it was found at an Alabama reservoir that is the sole drinking water source for about 350,000 people.The bomb was reportedly discovered during routine maintenance.Mobile Area Water and Sewer System (MAWSS) Director Bud McCrory says the discovery is "an unprecedented threat" to the drinking water."We are fortunate that this device was discovered before it could cause serious damage to our water supply or harm to individuals," he said.The bomb is being described