Musk didn't say where the breakthrough happened, but he recently announced he'll be giving a public tour of his latest tunnel project, which is under construction beneath the city of Hawthorne.
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Musk envisions building an entire system of underground tunnels beneath Southern California, whisking traffic on electric skates at speeds up to 150 mph.
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Musk's public tour of his Hawthorne tunnel is scheduled to happen on Dec. 10.