Musk's tweet was enticing enough, but he provided few details, and a spokesperson for the Boring Company declined to comment on the project. Big announcement, bigger plans: Elon Musk. Credit:Bloomberg It prompted a bombardment of follow-up questions and a great deal of skepticism. Who would pay for it? How long would it take to build? How would it be built? There were no answers. Nor was there any indication of just who in the government had given the plan the "verbal" green light. It's not clear what Musk is doing with these Twitter announcements. Such an ambitious project would require billions of dollars in funding and extensive approvals from federal, state, and local authorities.

The tunnel would be more than twice as long as the current record holder: the Gotthard Base Tunnel, a rail line that runs through the Swiss Alps. For some urban context: a recently opened stretch of subway in New York cost $US4.5 billion for some 3 kilometres of rails. It was first proposed in 1919 and opened to the public in January 2017. These things take time. The US Department of Transportation referred a query about the project to the White House, which said in an emailed reply through a spokesman: "We have had promising conversations to date, are committed to transformative infrastructure projects and believe our greatest solutions have often come from the ingenuity and drive of the private sector." Even if the Boring Co. did receive some kind of approval to start digging a tunnel, it's not clear how quickly the company will be able to move. Musk began digging in May on a small test tunnel using a second-hand boring machine, called Godot, which he acquired for his speculative new enterprise. Here's how Musk described the Boring Co. at a Ted Talk in April: "This is basically interns and people doing it part time. We bought some second-hand machinery. It's kind of puttering along, but it's making good progress."

In his Twitter replies on Thurday, Musk let slip a few more thoughts on his new mega project. Musk's planned 29-minute trip is considerably shorter than the current options. A drive from Washington to New York can take about five hours. Amtrak's Acela, its high-speed counterpart to regional train service, cuts the time down to about two hours and 45 minutes. A nonstop flight from Kennedy Airport in New York to Ronald Reagan National Airport in Washington is currently the fastest option, at one hour and 15 minutes. Musk-watchers have heard this before. Musk, a serial entrepreneur who was a co-founder of PayPal and the electric car company Tesla Motors, first unveiled his idea for a similar plan in 2013 when he unveiled a project for a high-speed system that would take people to San Francisco from Los Angeles in 30 minutes. That would take a speed of more than 1000 kilometres per hour.

On Instagram earlier this year, he posted a tunnel test run at just 200 kilometres per hour. Reuters reported that engineers in the next few weeks will conduct a crucial test of the Hyperloop concept, an idea that would have passengers and cargo packed into pods and sent through an intercity system of vacuum tubes. Hyperloop One, the Los-Angeles-based company developing the technology, is gearing up to send a 8.5 metre long pod gliding across a set of tracks, Reuters quoted a spokeswoman, Marcy Simon, as saying. Simon did not immediately respond to an email on Thursday. Musk launched the Boring Co. in December with a series of Twitter posts, tapping veteran SpaceX engineer Steve Davis to oversee the project.

In January, the company began digging in a parking lot as part of a planned network that Musk says will eventually cover all of Los Angeles. On Thursday, Musk said the East Coast tunnel will progress "in parallel" with the West Coast project. "Then prob LA-SF {LA to San Francisco] and a TX [Texas] loop," he wrote. Whatever this new "verbal approval" means, all those posts on Twitter are probably little more than an attempt to generate interest in future Boring Co. projects.

Musk himself suggested as much when David Lee, a Silicon Valley reporter for the BBC, questioned Musk's promises: "Verbal? Not on the dotted line? Seems premature to announce ... unless you're drumming up support for the project?" The New York Times/Bloomberg