Crater Lake in San Francisco East Bay?

We are out on a slightly rainy Sunday, hiking from Coyote Hills Regional Park to Don Edwards National Wildlife Refuge. It is an absolutely perfect time to hike this area. Cool and pleasant, the bay waters steel gray, dotted with ducks. Dark and linear clouds overhead, with light streaks. A few ground squirrels around. An egret leaves a muddy track on the shallow bay bottom. The Coyote Hills end has a few cars. Not much water in the freshwater marsh, big irregular polygons in the mud, separated by deep cracks. Most now under two or three inches of water, so the last rains have put some water into Alameda Creek. As we move south towards Don Edwards, we see the Dumbarton Bridge off to the right, and in the distant foreground, the outlines of a dozen or so salt cars, ready for their load. An aside, we've been getting some unattributed taking of blog content so inserting © 2009 backpack45.com Nearing the highway, a massive crater comes into view, maybe 200 feet deep. We can't quite see the bottom on the way south, but on the return trip we take a trail choice that leads us closer, so the lake on the bottom is visible. This has got to be well below sea level. Is that water salt or fresh? This is the first time at Don Edwards for us, and a pleasant surprise. Old fishing cabins, great views.