North Korea's Internet was on the fritz for a second day Tuesday. But the US is staying silent on whether it launched a cyber attack as payback for the hacking of Sony Pictures.

And in the murky world of cyber security, experts say there are several plausible scenarios for why North Korea suddenly went dark, stressing it's impossible to know exactly what happened.

Google on Monday announced that the first completed prototype of its self-driving car is ready to be road tested.

"We're going to be spending the holidays zipping around our test track, and we hope to see you on the streets of Northern California in the new year," the Internet titan's autonomous car team said in a post at Google+ social network.

Trying not to spill your coffee while wedged up against perfect strangers on a jam-packed subway is a daily challenge for New York commuters.

Now one real estate executive, inspired by an Alpine holiday in France, believes that cable cars can solve New York's transport problem -- too many people and too few trains. 

The United States urged North Korea on Monday to admit it ordered a cyberattack on the Hollywood studio Sony Pictures and to pay for the damage it had caused.

Twitter users anywhere in the world can control the lights on a holiday display in New Jersey.

Tweets will turn on a 9-foot (3-meter) Christmas tree, menorah and over 1,000 LED lights at Oxford Communications in Lambertville and also turn them off.

Italy's competition watchdog slapped TripAdvisor with a 500,000 euros ($613,000) fine on Monday for failing to warn users some of the opinions posted on the popular review website may be fakes.

TripAdvisor adverts emphasize "the authentic and genuine nature of the reviews, persuading consumers to believe the information is always reliable and reflects real tourist experiences," the AGCM said in a statement.

The angular vessel looks like a waterborne stealth fighter. It rides atop underwater torpedo-shaped tubes powered by a pair of 2,000-horsepower gas turbine engines. Gyroscopes keep the ride smooth.

Sadly, Ghost is all revved up with no place to go. The brainchild of a wealthy inventor and entrepreneur, Ghost might never be a familiar household name like Humvee or Apache — even if it works as advertised — because its creator built a warship the Navy isn't convinced it needs.

New York City's subways — the biggest U.S. mass transit network — serve more than 6 million daily riders who depend largely on a signal system that dates back to the Great Depression of the 1930s.

Antiquated electro-mechanics with thousands of moving parts are still critical to operations. Dispatchers still monitor most trains from 24-hour underground "towers," and they still put pencil to paper to track their progress.

The Sony hack, the latest in a wave of company security breaches, exposed months of employee emails. Other hacks have given attackers access to sensitive information about a company and its customers, such as credit-card numbers and email addresses. One way hackers can sneak into a company is by sending fake emails with malicious links to employee inboxes. Here are five simple steps to make your email more secure and limit the harm a hacker can have:

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South Korea and Japan on Saturday vowed to work closely with the U.S. to combat cyber crime, after Seoul blamed North Korea for a crippling cyber attack on Sony Pictures.

South Korea said it would share with Washington information "related to the cyber attack on Sony," which it said bore all the hallmarks of an onslaught on its own banks and media agencies by the North last year.