 Complete the following phrase:  The San Francisco Bay itself remained undiscovered by Europeans until members of the Portolá expedition, while trekking along the California coast, encountered it in 1769 when the bay blocked their continued journey north. In 1806, a Spanish expedition led by Gabriel Moraga began at the Presidio, traveled south of the bay, and then east to explore the San Joaquin Valley. In 1821, Mexico 