In 2016, entrepreneur Elon Musk launched the Boring Company with the goal of one day constructing a large urban network of tunnels.

The company is working on two projects in the Washington, DC region and Los Angeles. It first broke ground on a test site in LA in late 2017.

In order to bore tunnels, construction crews will need to excavate through rock and soil.

On Monday, Musk tweeted that instead of disposing that leftover earth, he plans to recycle it into interlocking, Lego-style bricks that can be used to build houses and other structures.

When asked how long the construction of a building using the bricks would take, Musk replied that "two people could build the outer walls of a small house in a day or so." It's unclear how much the bricks would cost.

The first kit will be ancient Egypt-themed and will be available around May, according to Musk.

The Boring Company's site had already hinted at these plans before Musk's tweet, and acknowledges that using rock for bricks is not a new concept.

"Buildings have been constructed from Earth for thousands of years including, according to recent evidence, the Pyramids," the site reads on its FAQ page. "These bricks can potentially be used as a portion of the tunnel lining itself, which is typically built from concrete."

A growing number of startups have developed similar systems. In 2016, French architecture firm Multipod Studio unveiled its PopUp House, a customisable home made from stackable blocks that can be built in about a month.

Indian architect Anupama Kundoo has also developed low-cost, plaster bricks that can resist earthquakes.

The Boring Company's website claims that creating bricks would reduce both the tunnelling costs and the environmental impact of its projects (since cement production accounts for over 4 percent of global CO2 emissions).

Musk, who also founded SpaceX and Tesla, says his underground tunnels would serve as transportation corridors that alleviate traffic. The company is still billions of dollars and years away from building its first official tunnel, however.

In LA, California's Environmental Quality Act regulates big construction projects, and Curbed reports that the review process alone could take at least three to four years.

Critics of the Boring Company's vision, including a number of transportation analysts, argue that improving mass transit systems would be a more efficient (and likely less expensive) strategy toward decreasing traffic.

Some transportation experts say that Boring's tunnels would create even more gridlock.

On March 9, Musk tweeted that he is "adjusting the Boring Company plan: all tunnels & Hyperloop will prioritise pedestrians & cyclists over cars," adding that the systems "will still transport cars, but only after all personalised mass transit needs are met. It's a matter of courtesy & fairness. If someone can't afford a car, they should go first."

Boring's first tunnel system would have "thousands of small stations the size of a single parking space" that "blend seamlessly into the fabric of a city," Musk tweeted.

This description sounds a lot like a subway, except with much smaller stations and futuristic vehicles that would shepherd passengers to their destinations.

This article was originally published by Business Insider.

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