The enchanting coastline between San Francisco and Pacifica harbors its secrets, one of which recently caught the attention of Francisco Alvarado and his loyal Chihuahua, Little Bean.
A curious tunnel emerging from the cliffside piqued their interest during a routine beach walk, sparking questions about its origin and purpose.
To investigate this peculiar formation, Alvarado sought insights from geology professor Katryn Wiese at the College of San Mateo, who frequently conducts field trips to the area.
This section of coastline, characterized by its soft rock, is prone to landslides, and the beach’s appearance varies drastically with the changing seasons.
Wiese explains, “The sand piles up in the summer, and then throughout the winter, the heavier waves start excavating it,” which exposes a wealth of geological treasures.
Spring, in particular, is an ideal time when the rocky cliffs reveal a collection of fossil-rich formations, with clam fossils dating back nearly a million years found near the cliff’s base.
During low tide, Wiese and Alvarado embarked on a search for the mysterious tunnel, noting the wood pilings described by Alvarado and heading towards a cliff that juts out into the ocean.
Upon arrival, they spotted the tunnel’s dark entrance amidst the layers of rock.
Wiese explained the geology of the area, emphasizing how resistant rock formations stand out as waves erode softer materials.
The cliff presented a layered appearance, resembling a cake, with the tunnel carved into a softer middle tier.
As they approached the entrance, they observed the mouth had partially collapsed, leading them to ponder the safety of entering the tunnel.
Despite initial reservations, Alvarado decided to explore further into the tunnel, which quickly narrowed as he made his way inside.
The tunnel’s interior featured smooth walls, suggesting a natural formation rather than a man-made one, as he crawled deeper into the darkness.
Wiese suspected the tunnel’s existence might correlate with recent geological activity and wanted to verify her hypothesis against historical photographs.
Reviewing her archives, she confirmed the tunnel’s emergence had no record prior to 2021, indicating that the tunnel likely began to form between 2022 and 2023.
The smoothness of the tunnel was attributed to wave action during a recent storm season, which accelerated the excavation process in the softer rock.
Wiese noted that once an indentation forms, the ocean would continuously deepen it as long as the conditions allowed.
Photographic evidence showed the first traces of the tunnel in the earlier months of 2021, revealing a previously unremarkable rock face that had gradually evolved.
Contrary to popular speculation, the tunnel was not an old artifact from the 1800s or a hidden passage for illicit trade.
The smooth rocks, rounded walls, and flat roof pointed towards the forces of nature, with waves contributing to its formation rather than human intervention.
Despite marks that resembled the work of human tools, these likely resulted from curious visitors rather than deliberate excavation of the tunnel itself.
Ultimately, Wiese concluded that the tunnel is an impressive demonstration of the changing dynamics of coastal erosion rather than a relic of history.
With these insights, Alvarado’s initial thrill of discovery now is tempered with an appreciation of the natural world’s ability to evolve and transform.
Next time he and Little Bean visit the beach, they might find a dramatically different landscape, reminding us all of the ever-changing nature of coastal environments.
image source from：https://www.kqed.org/news/12041355/what-is-that-massive-tunnel-on-the-beach-south-of-fort-funston
