From Halopedia, the Halo wiki
The Theran Crown is a cryovolcano on the Unified Earth Government-controlled planet Neos Atlantis. It is described as gloomy and spire-studded and is ringed by cliffs of green ice. The green color could indicate the presence of some sort of algae-like organisms in the ice, but could also be due to other impurities, such as sediments or air bubbles. The Theran Crown can be easily seen from the UNSC Recreational Facility 6055-NA-A in the Liberty District.