Prosecutors claim that the collapse of the Cologne city archive in 2009 was caused by mistakes in construction of a new U-Bahn line, an accident which they argue was caused by human negligence.

The archive was built in 1971 using groundbreaking techniques to assure that historical documents could be preserved in a cool environment. Upon completion it became the store for archival material, some of which dated back to the Middle Ages.

But it collapsed suddenly on March 3rd 2009, pulling down two adjacent buildings with it. While construction workers were able to alert archivists and visitors to the building, two young men who were sleeping in the adjacent buildings were buried in the rubble and died.

Most of the archival material could eventually be recovered and repaired.

Prosecutors charge that the collapse was caused by a building error during construction of an U-Bahn station under the archive, which occurred in 2005.

Lead prosecutor Torsten Elschenbroich described on Wednesday how the builders came across an enormous boulder when they were trying to hollow out the space for the station. The boulder lay in an area of the site where a concrete wall was to be built. Repeated attempts to break it apart failed, so under immense time pressure, the foreman decided to build around it.

According to Elschenbroich, the foreman neither reported the boulder to his seniors nor recorded it in the construction log book.

The decision proved to be fatal though, according to prosecutors. Pressure built up at the spot where the boulder was until, four years later, the structure couldn't take the weight anymore, and water gravel and earth broke through and started flooding into the construction site.

Alongside the foreman in court on Wednesday were four more defendants, some employees of the construction company, others city officials. 

Prosecutors charge that they failed to spot the mistakes which led to the collapse. Elschenbroich said there had been “countless failures” in the oversight of the project.

The building company has denied that it could have foreseen the collapse though. It argues that it came about due to a “hydraulic heave”, a natural occurrence which no one could have been prepared for.

Given that it has taken almost nine years for the case to come to trial, there is considerable time pressure on the prosecution to put their case across and convince the judges. Charges of negligent homicide carry a 10-year statute of limitations, meaning that if no judgement is released by March next year, the case will have to be dropped.