Castle Rock National Wildlife Refuge is a part of the National Wildlife Refuge system, located in Crescent City, California. It is located a half mile off-shore, close to the Del Norte County Regional Airport. At only 14 acres, it is a small but important National Wildlife Refuge, and is administered as part of the Humboldt Bay National Wildlife Refuge, located 100 miles to the south.
Castle Rock is totally closed to the public, and even if it were not, access to the island would be very difficult, as it is basically sheer rock faces rising from an ocean with strong currents. It is not large enough to have any trees, although parts of it are covered with grass. The island is part of the National Wildlife Refuge system because it is home to the common murre and tufted puffin, hosting over 100,000 breeding pairs. It is also home to other birds, resident and migratory, as well as being a haul out point for sea mammals.
What is most interesting for me is that despite the vast amounts of natural and wilderness areas in Northern California, this single rock, around one-fiftieth of a square mile, is one of the few offshore islands that provides habitat for nesting shorebirds, and serves as a keystone to an entire ecosystem, despite being probably unknown to even people in the area.
https://www.fws.gov/refuge/Castle_Rock/