There are many natural arches and tunnels of the sort up and down the coast. This isn't one of them - it's man-made.
Before the bridge and Highway 1 were built here, Pescadero Creek emptied out through a marsh, part of which remains. In drought years, the creek mouth would become blocked by a sand bar by the end of the dry season. When the first heavy storms of the winter brought floods down from the hills, the creek would back up through the marshes and upstream to the town of Pescadero, until the accumulating waters finally blew a hole through the sand bar.
This area was once farmed more heavily than today, and fields came right down to the marshes on both sides of the creek. Farmers got tired of having their crops and homes flooded out repeatedly, and after a particularly bad episode in December 1898, they banded together and raised $200 to have a hole cut through the rock and bypass the sand bar. According to Ron Duarte of Pescadero:
“A bunch of old timers made that tunnel. It is not natural. Everybody thinks it is natural but it is not. They thought [the tunnel] was going to keep the mouth of the creek open. Maybe it did and maybe it didn’t. But, most of the time I don’t think it did much good. That was man-made, that tunnel.”
The tunnel remains as a curiosity, but is mostly ineffective. Local records show that the sand bar has closed the creek numerous times since, and it was sometimes opened with bulldozers around the time of WWII. Indeed, one of the local State Park rangers has told us that the bar may still open 'coincidentally' a day or two before the steelhead run and fishing season begins.
Do NOT attempt this as a drive up. There is high speed, often heavy traffic on the bridge approach. Instead find the parking spot just south of the bridge, descend the wooden stairs to the roadside walkway, and enjoy a stroll to the cache and back. (See extra wayponts for parking entrance and stairs.) Please rehide so the cache is not visible to passing foot traffic.