Buried Horizon

In the vast Mongolian steppe, geologist Arslan Tömör discovers a mysterious sinkhole revealing an ancient underground city called Ürgelkh ("Eternity"). Guided by Sarangerel, a teenage girl whose prophetic dreams foretold the city's emergence, and joined by Chinese archaeologist Dr. Chen, Arslan descends into a perfectly preserved technological marvel built 15,000 years ago.
They discover Ürgelkh is both sanctuary and failsafe mechanism-an ark containing thousands of suspended humans (Dormants) preserved from Earth's last great ecological collapse. The city begins awakening these ancient sleepers while simultaneously initiating a programmed "Implementation"-a controlled reset of Earth's damaged ecosystems that will disrupt modern civilization but prevent total collapse.