Complex sentence: Prior to that escalation, Frei Otto, one of the architects responsible for the project, cited a 2003 report for calling for a halt to the project, saying the ground in the area is too unstable for large scale underground works.
Split into simpler sentences: Prior to that escalation, Frei Otto, one of the architects responsible for the project, cited a 2003 report for calling for a halt to the project. He alleges the ground in the area is allegedly too unstable for large scale underground works.