Tesla Motors is set to make a big announcement tomorrow (Oct. 9), after more than a week of speculation about the electric car company's next move, sparked by a mysterious tweet from CEO Elon Musk.

On Oct. 1, Musk wrote in a post on Twitter that it was "time to unveil the D and something else," with a teaser image of a car in a shadowy garage. The message has since been retweeted more than 14,000 times.

It's still not clear what "the D" is (though Musk later joked in a follow-up tweet that it is not as intimate as some giggling corners of the Internet thought it was). But investors are excited nonetheless. In the past week, Tesla's shares have gone up nearly 6 percent, CNN reported, rising to their current value of $257.98 before today's closing bell.

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Citing anonymous sources, The Wall Street Journal reported that Tesla — which is based in Palo Alto, California — would be revealing more computer-controlled driving features for its Model S electric sedan. Another report in Bloomberg News, similarly suggested that "D" could stand for driverless.

Autonomous driving features would certainly fit into Musk's long-term plans. In addition to creating space taxis and a hyperloop transit system, Musk has had his heart set on creating driverless cars. The prolific billionaire inventor recently told CNN his goal was for Tesla to be "90 percent autonomous" in 2015. And he told The Wall Street Journal in September that the technology to make driverless cars would be ready in five or six years.

But Green Car Reports, Jalopnik and others have also speculated that the D could be an all-wheel drive option for the Model S (something Tesla had been expected to add to its sedan for a while). One analyst told MarketWatch that he expects Telsa to unveil both semi-autonomous driving features and an all-wheel drive version of the Model S.

We won't know for sure until the big reveal, which is set to take place tomorrow at 7 p.m. PDT at the Hawthorne Airport in Los Angeles County, California, right by the headquarters of Musk's other major venture, the private spaceflight company SpaceX.

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