The outage began about 11:10 a.m. PST after a construction crew severed a fiber-optic cable, said a representative for WorldCom, the parent company of UUNet.
The problem slowed or stalled altogether Internet service to UUNet customers in San Francisco and Santa Clara, Calif.--the heart of Silicon Valley.
By noon, most customers regained service after UUNet routed traffic to other parts of its network, the representative said.
The cable was still being repaired early Wednesday afternoon. The representative didn't know how many customers were affected.
Networks managed by UUNet carry 70 percent of the world's e-mails and 40 percent of its Internet traffic.