A muddy exception to the glistening drive down California’s Highway 1, Elkhorn Slough is the improbable and modest host to more than 700 animal and plant species. These mere seven miles of salt marsh are home to a diverse crew of southern sea otters, seals, sea lions, endangered local salamanders, and California red-legged frogs.

The Slough is also an important stop on the Pacific Flyway route, which migrating birds take every year from Alaska to Patagonia. Tens of thousands of birds pass through Elkhorn Slough annually, and more than 340 avian species have been counted around the marsh, enough for the American Birding Conservancy to designate the site a Globally Important Bird Area.

But much of what makes Elkhorn Slough a natural wonder is invisible. Wetlands like these soak up contaminants before they can infiltrate larger bodies of water, and they even absorb greenhouse gases that could otherwise float into the atmosphere. These unassuming wetlands are the remnant of the river that once drained California’s vast inland sea until that function shifted north to what is now San Francisco Bay.

This understated estuary might not catch your eye next to the glowing Pacific or the green, muscular hills flanking the ocean. But should you find yourself cruising the California coast, make some time for this murky marvel that is a mini ecosystem unto itself.