Sources:
https://boringcompany.com/
http://www.theverge.com/2017/4/28/15476268/elon-musk-the-boring-company-car-tunnel-concept-video
Elon Musk has been in the news for a number of reasons this week, although this one is pretty boring
Personally I think this could be a cool idea as when underground cars wouldn't be driven so much as transported so getting across a city could be much faster, and any congestion taken off the road is good imo.
Elon tweeted this in December, with many thinking that he was just joking around:
However this image taken outside Tesla shows that Elon is actually serious about this and that work has already begun:
And then Musk tweeted a link to boringcompany.com which as of today just shows the concept video.
From anybody else I would think this idea is unrealistic, but he seems to have a record of pulling these things off ?