The grand estuary of northern California from the Sierra to the sea, with its rivers, delta and bay, is the ecological heart and soul of our region, and yet many of us have little idea how it works and who lives in its mysteriously murky water. To learn about the surprisingly diverse denizens of its depths, we descend beneath the waves in our street clothes at the Aquarium of the Bay in San Francisco’s Fishermen’s Wharf. We see colorful creatures of many kinds up close, swimming beside and above us and all part of a tightly woven web of life. Giant sea bass, sharks, sardines, our state fish and a clever octopus make their appearances and help us understand their complicated and vulnerable world a bit better. Then we go outdoors to visit sea lions who found a refuge in 1989 on boat docks at Pier 39 and have never left. The entertaining and engaging sea lions are now San Francisco’s number one visitor attraction and amazing ambassadors for their watery world which we need to protect. Then, we journey to Sausalito to see the entire estuary and how it works on foot from the Sierra all the way to sea, indoors and in miniature. The Bay Model was built by the Army Corps of Engineers in 1957 to test outrageous plans to dam and fill large portions of the bay. For three decades, this 1 to 1,000 scale model provided state-of-the-art scientific understanding of precisely how the estuary’s hydrology works and is now open for us to visit to learn about the most important estuary in the western hemisphere in hopes we’ll choose to manage it wisely for the sake of all of us who depend on it for our survival.