MOUNTAIN HOUSE, Calif.  This town, 59 feet above sea level, is the most underwater community in America.

Because of plunging home values, almost 90 percent of homeowners here owe more on their mortgages than their houses are worth, according to figures released Monday. That is the highest percentage in the country. The average homeowner in Mountain House is “underwater,” as it is known, by $122,000.

A visit to the area over the last couple of days shows how the nationwide housing crisis is contributing to a broad slowdown of the American economy, as families who feel burdened by high mortgages are pulling back on their spending.

Jerry Martinez, a general contractor, and his wife, Marcie, an accounts clerk, are among the struggling owners in Mountain House. Burdened with credit card debt and a house losing value by the day, they are learning the necessity of self-denial for themselves and their three children.