HAWTHORNE, Calif. (AP) — Billionaire Elon Musk has released a photograph of a tunnel he's building under a Los Angeles suburb to test a novel transportation concept for a system that would move people underground in their personal cars rather than by subway trains.

The founder of SpaceX and Tesla tweeted during the weekend that the tunnel was 500 feet so far and should be 2 miles long in three or four months.

In August, the Hawthorne City Council granted a permit allowing an underground extension of approximately 2 miles from SpaceX property, crossing under a corner of the municipal airport and beneath city streets to a point about a mile east of Los Angeles International Airport.

Musk also tweeted that hopefully in a year or so the tunnel would stretch along the Interstate 405 corridor from LAX to U.S. Highway 101 in the San Fernando Valley, which would require approval from other governments. That span is about 17 miles.

Musk has complained about what he called "soul-destroying" Los Angeles traffic. He added The Boring Company to his ventures, acquired a tunnel-boring machine that had been used in a San Francisco Bay Area project and put it down a shaft in a SpaceX parking lot this year.

Hawthorne council document say the "Test Tunnel for Zero Emission Subterranean Transportation" has an exterior diameter of 13.5 feet (4.1 meters) and an interior diameter of approximately 12 feet (3.6 meters) and will run as deep as 44 feet (13.4 meters) beneath the surface.

"When the project is completed, the Test Tunnel would house a 'skate' system that would be tested to prove the viability for transporting pedestrians or personal vehicles. The concept is that a vehicle would be drive on to the skate, the engine would be turned off and the vehicle and its passenger would be transported from one end of the Test Tunnel to the other," the August resolution said.

"The Test Tunnel project would involve SpaceX engineers repeatedly testing and experimenting with personal vehicle types suitable for placement on the skates; refinement of the design and technology; and general data collection on performance, durability, and application. No public use of the Test Tunnel would occur, and no people would be occupying vehicles located on the skates as the skates are tested within the tunnel," it added.

Construction was expected to take about five months to complete, the resolution said. Musk has maintained that tunneling can be accomplished much more rapidly than occurs with current methods.

The plan allows the city to request that the tunnel be filled in when testing is complete.

Musk has also advocated another transportation concept called the "hyperloop," a network of nearly airless tubes that would speed special capsules over long distances at up to 750 mph (1,207 kph), using a thin cushion of air, magnetism and solar power. SpaceX has recently hosted competitions by development teams on a test track built at its headquarters.

On Monday, SpaceX conducted its 16th Falcon 9 rocket launch of the year, carrying a South Korean satellite into space from Florida's Kennedy Space Center. The rocket's first-stage booster scored another successful landing aboard a floating platform in the Atlantic.