Elon Musk’s Boring Co said on Monday it raised $112.5 million in equity, with 90 percent coming from Musk himself, as the company seeks to build underground tunnels for its hyperloop transportation project.

In a regulatory filing, the company listed 31 unnamed investors as participants in the funding round. Besides Musk, the rest of the investors were early Boring employees, according to the company.

Musk, who also leads electric vehicle maker Tesla Inc (TSLA.O) and rocket company SpaceX, is seeking to revolutionize transportation by sending passengers packed into pods through an intercity system of giant, underground vacuum tubes known as the hyperloop. Forbes magazine estimates Musk’s net worth at $19.3 billion.

The funding comes about five months after Musk said Boring Co would compete to fund, build and operate a “loop” to connect downtown Chicago with O’Hare Airport.

Musk earlier sold Boring Co hats and flamethrowers to raise funds.

The company says tunnel-digging projects can cost as much as $1 billion per mile, but its goal is to lower these costs by a factor of 10 or more.

In October, the Maryland Department of Transportation gave “conditional approval to the construction of a tunnel from Baltimore to Washington” for the project by issuing a utility permit.

Last month, six Democratic members of Congress from Maryland and Washington wrote to Maryland Governor Larry Hogan seeking information about the state’s support for the proposed Hyperloop Project, including the regulatory standards that will apply.

“While the Hyperloop is an exciting project that has the potential to transform transportation along the entire U.S. East Coast, it is also a project that would utilize a wholly new technology and could have significant impacts on our constituents,” the members wrote.