The engines of God

Enjoyable space opera, with a mystery- why did advanced civilizations on several planets suffer catastrophes that plunged them into chaos, death and sometimes rebirth as a primitive civilization, and why is there a huge stone fake city (Oz) on a nearby planet- what was its purpose? A team of scientists and archeologists tries to finish their work as the terraforming deadline looms on a planet chosen as a haven for select people from a devastated Earth. They discover something that prompts an exploration of another planet, which prompts yet another exploration. The team takes many risks, and several members pay the ultimate price, but the mystery is solved in the end.