Police evacuated about 15 homes in a quarter-mile radius before the operation, Bay Head Police Sgt. Todd LaRue said. The Coast Guard kept boats clear of the area.
An ordnance disposal team assigned to a naval station in Colts Neck examined the device after it was discovered Tuesday night, and decided the only safe thing to do was to detonate it in place at high tide the next day.
The device was believed to be a moored contact mine from either World War I or World War II, base public affairs officer Michael Brady said. The round mines contain spikes designed to detonate upon contact.
Brady said the team suspects only the 10-pound C-4 charge of explosives used to detonate the device exploded. The team was taking a piece of metal that survived the blast back to the base to try to determine what the device was and where it may have come from.
During World War II, German U-boats patrolled the ocean just beyond the horizon and targeted merchant ships as they sailed along the coast. Superstorm Sandy last year may have exposed the device, Brady said.
The explosives team has responded a half-dozen times to similar incidents since the storm hit the Jersey shore on Oct. 29, but mainly to the north near Sea Bright and Sandy Hook, which was used as a military proving ground for explosives between 1874 and 1919.
While startling to some, the discovery of old munitions on Jersey shore beaches or just offshore is not all that unusual. The military has said it dumped large quantities of munitions overboard at the end of both World Wars as the conflicts were ending.
In 2007, more than 1,000 pieces of World War I and World War II-era munitions were discovered on the beaches of Long Beach Island after they were unwittingly sucked up by dredges and pumped ashore as part of a massive beach replenishment project.
Similar discoveries have been made off the coasts of Delaware and New York.
In 2007, the U.S. Army Corps of Engineers recovered 1,080 pieces of material, including fuses and other military hardware, from Surf City and Ship Bottom that were unearthed as part of the beach project.
Merchants rushed to capitalize on the unwanted publicity by selling T-shirts with slogans including, "Our Beaches Will Blow You Away" and "I Got Bombed On Long Beach Island."
