We've heard of underground restaurants in places like New York, but had no idea they existed in San Francisco. Restaurants here must meet numerous legal criteria, including inspections by the health department, liquor-control board, and fire department. Therefore, this entry must be somewhat vague. But we can tell you that Mamasan's has been in operation for four years, serving more than 1,000 customers. It's not really a restaurant, since it's housed in the Mission District apartment of 37-year-old Lynette. There are no advertisements or signs. Like other underground eateries, the best (and probably only) way to find it is to get invited by a previous diner. It's a family-run operation, with Lynette's 61-year-old mom (the namesake mamasan) running around the kitchen smoking a joint. And by the way, the food's damn good. For reservations and other details, hire a private investigator.