Who Built it?
There´s a story or have they any origin?
Human analysts of the 26th century believed that an extensive collection of species were brought to the Installation 05 (aka Delta Halo) to be studied by the Lifeworkers as part of the Conservation Measure. It is possible that some of these species were intelligent and built these ruins. A plausable reason as to why we never see any bodies is that all species got consumed by the Flood and eventually became the Gravemind.