When a sizable chunk of concrete fell from the roof of the Bay Bridge tunnel into the path of an unsuspecting motorist last month, we were told it was caused by an isolated bit of corrosion. Now, unfortunately, we learn it may be the first sign of a much larger problem.

The Chronicle reports that a basic inspection by Caltrans on the eastbound tunnel discovered 12 more sections in danger of separating from the arch, potentially falling into the roadway. Determining the full extent of the damage, which is likely caused by rain water leaking through from the roadway above, will require specialists with x-ray machines and millions of dollars.

The corroded areas spotted by workers were discovered by a relatively low-tech method: Banging the wall with a hammer and trying determining whether it sounded hollow. More sophisticated tests have yet to be conducted, but one Caltrans engineer, Ken Brown, says not to worry.

“We don’t think we have a big issue right now," said Brown, "But until we finish our investigation, we don’t know anything for certain.”

The 3-inch-thick chunk of tunnel that fell away in January measured two-and-a-half square feet. It landed in the path of a Ford Fusion driven by an elementary school teacher. The car ran over the concrete, resulting in roughly $3,000 in damage to the vehicle.

The tunnel in question was completed in 1936 and hasn't undergone an inspection of this nature since 2004. It seems the new eastern span isn't the only section of the bridge in need of some TLC.

Previously: Now The Bay Bridge Tunnel Is Falling Apart Too