Earth : North America : United States of America : California : Bay Area (California) : South Bay
The South Bay is the region of the Bay Area around and south of the San Francisco Bay. No legal or geographic border precisely defines the point at which you leave the Peninsula and enter the South Bay, but it's safe to say that the Peninsula comprises Palo Alto and points north, while Mountain View and points south are in the South Bay. Similarly, as you travel south in the East Bay, you enter the South Bay upon entering Milpitas.
The South Bay is sometimes described as Silicon Valley due to the number of famous technology companies that are located there.