Nobody was hurt, no oil spilled, and damage to both ships was light.
Still, officials from Washington to Tokyo are scratching their heads and asking how, despite 21st-century technology, a high-tech U.S. Navy submarine could have hit a Japanese supertanker.
Actually, sources familiar with the U.S. submarine fleet say, it's probably not that hard. If anything, submariners say, it's remarkable that windowless, bulky submarines, operating in the crowded waters of the Strait of Hormuz, don't run into trouble more often.
"People would be surprised. It's easier to hit things than you'd imagine," said a U.S. Navy source, who asked not to be identified. "That's a very busy, narrow piece of waterway."
The basics of the accident: The USS Newport News was traveling underwater in the Persian Gulf at 10:15 p.m., local time, when its bow hit the stern of the Japanese supertanker Mogamigawa.
Neither hull was breached. The Navy source told ABC News that it was possible the Newport News had been trying to surface. That could not be confirmed, though.
The Newport News is a Los Angeles Class attack submarine, part of a U.S.-led multinational task force patrolling the Persian Gulf. It has a crew of 127. The tanker had a crew of 24.
The sub is nuclear-powered, but the Navy said there were no nuclear weapons onboard.
A Navy statement said the submarine had been conducting "Maritime Security Operations" -- MSO for short -- intended to "deny international terrorists use of the maritime environment as a venue for attack or to transport personnel, weapons or other material."
The Strait of Hormuz is important and sensitive, because 40 percent of the world's oil supply pass through its waters.
Officially, the Pentagon would not say much more.
"I can't speculate. That's why we have investigations," said Lt. Cmdr. Keven Aandahl of the Navy's 5th Fleet in Bahrain.
Tetsuzo Fuyushiba, the Japanese transport minister, was also circumspect.
"I want more information to be collected," he said to reporters in Tokyo. "I was relieved to hear that there was neither human injury, nor oil leakage from the tanker load, but I gave an order to remain alert and to collect more detailed information concerning this issue."
Sources said there were several factors that probably contributed to the crash.
The submarine's radar was probably turned off, and its sonar was most likely in a "passive mode" -- set to pick up echoes from other vessels, but not to send out any signals of its own. Both settings would have helped make the Newport News harder for potential enemies to find.
"You're where you don't want people knowing where you're at," the source said.
In crowded waters, submarines are also in a bit of a squeeze. If submerged, they need to be deep enough to avoid ships above them, but they also have to be careful not to hit bottom in the relatively shallow straits at the mouth of the Persian Gulf.
Two years ago, another American submarine, the USS San Francisco, ran aground in the Pacific about 350 miles south of the island of Guam. It was reported to be at a depth of more than 400 feet, cruising at a speed of 35 knots. One crew member was killed, and 23 were injured.
An investigation found later that the undersea mountain it had hit was not clearly marked on the maps the submarine carried.
In that accident, 20 crew members were given medals or other commendations for their quick reactions.
The skipper, however, was reprimanded and relieved of his command.
ABC's David Kerley contributed to this report.