Stumbled upon the Mussel Rock Tunnel reading the CHPW volumes for 1956:
"The Mussel Rock Tunnel was built at the behest of Hibernia Bank co-founder Richard Tobin and was completed likely by January 1875. Specifically, the Mussel Rock Tunnel was built to permit stage travel between Tobin's home at Rockaway Beach and his family's home in San Francisco. The Mussel Rock Tunnel is often claimed to be the first highway tunnel in California. The Mussel Rock Tunnel was quickly replaced by the Half Moon Bay-Colma Stage Road which had been completed over Montara Mountain by 1879.
The Mussel Rock Tunnel was infamous for filling with sand during high tides and never truly worked properly as through highway. The Mussel Rock Tunnel was possibly as long as 90 feet in length. Ultimately much of the Mussel Rock Tunnel and the approach roads were destroyed by the 1906 San Francisco Earthquake."
I posted the CHPW article on the Gribblenation Facebook page:
https://www.facebook.com/72868503020/posts/pfbid02QrTS3demNswPf32YwTbRQvXHG3NhR5aXcWSSSW337GAMe3TgauH8AScyerFRru2Hl/?d=n
Also, I found this study which includes the Mussel Rock Tunnel starting on Page 72:
https://geog.sfsu.edu/sites/default/files/thesis/Shawn%20Heiser_Mussel%20Rock_Thesis.pdf
Hi Max: You can find the trace of this on the U.S. Geological Survey's map titled "San Mateo, CA" (15 minute series, surveyed 1892-94). This map is available for online viewing by Googling "Historic USGS maps," and following the instructions therein.