Elon Musk has a lot of ideas, and it isn't always clear whether he intends to make something real out of them.

Hence the cautious enthusiasm surrounding a photo he posted to Twitter on February 4.

It appears to show the interior of an industrial-sized drill used to bore tunnels. It resembles - but is not quite the same as - similar boring machines used for tunneling projects elsewhere. Examples include the German-built Gripper used for the Gotthard Base Tunnel in Switzerland and the "Bertha" mega-drill created for an ongoing smaller-scale project in the US city of Seattle.

The image, which had not previously been posted to the internet, implies the founder of Tesla and SpaceX has taken a very concrete first step toward fulfilling a "promise" to build a tunnel under one of the world's megacities, Los Angeles, population nearly four million.

Later that day, as if to show proof-of-concept, he tweeted a second picture of a Washington DC tunnel.

Musk has mentioned tunnels before as a theoretical solution to traffic problems plaguing the major cities of the world. But it took a moment of apparent frustration in December 2016 for him to commit himself online to the concept.

But the "promise" remained just that - until late January, when Musk appeared to commit to actual drilling in a Saturday morning tweet.

When one of his followers asked "Where will your tunnel be?" Musk responded with the following:

"Starting across from my desk at SpaceX. Crenshaw and the 105 Freeway, which is 5 mins from LAX"

The following week Musk told Wired magazine that, over the weekend, workers at SpaceX headquarters in LA had excavated a "test trench" roughly 30 feet wide (9.1 meters), 50 feet long and 15 feet deep.

"We're just going to figure out what it takes to improve tunneling speed by, I think, somewhere between 500 and 1,000 percent," he said, also telling the magazine he ultimately envisions tunnels "10, 20, 30 layers deep (or more)."

Musk added: "We have no idea what we're doing - I want to be clear about that."

On his Twitter profile, the inventor/investor now describes his professional interests as follows:

Tesla, SpaceX, Tunnels & OpenAI.

The obvious omission seems to be Hyperloop, the near-vacuum tube allowing magnetic trains to reach theoretical speeds of 1,200 kilometers per hour (760 miles per hour).

The omission leaves me wondering, however. To what extent is the Hyperloop concept dependent on "tunnels," and could the Hyperloop in turn become a subcategory of them.

Musk says it will be necessary to bore tunnels to make Hyperloop feasible, even if most Hyperloop conceptual art shows the dual vacuum tubes supported on pylons above the ground.

He has hinted as much on Twitter.

Finally, with regard to SpaceX and the colonization of Mars, one idea that's been floated is whether tunneling underground on the Red Planet would help protect astronauts from radiation and extreme cold.

So perhaps the technologies tested in LA should not be viewed as an isolated idea or project, but rather as just one more component in Elon Musk's comprehensive and ever-expanding vision of human travel and exploration.