Initially, the mine operated by extracting ochre at the surface and magnetite iron ore at depth, with some tin also being raised. Uranium ores were discovered near the surface, primarily in the form of hydrated minerals such as Torbernite, locally known as ‘Green Jim’. Deeper excavations revealed the presence of pitchblende, which became the primary ore mined until the mine’s closure in 1930.

Unlike other Cornish mines where uranium occurrences were sporadic, South Terras was distinctive for its significant lode, approximately 250 fathoms (457.2 meters) in length, where uranium derivatives were the primary ores. The entire site is highly radioactive, with radiation levels far exceeding normal background levels. While background radiation typically measures 10 to 12 counts per second, at South Terras, counts range from 50 to 200 counts per second. In some areas, counts surpass 500 counts per second, and at the head of the new adit shaft, contaminated water seepage causes counts to exceed 2000 per second across wide sections of the sunken floor.

Despite the hazardous radiation levels, the reasons behind the continued occupation of the house on the site remain unclear. The mineral list from South Terras excludes certain species like Xanthiosite, aerugite, and bismutoferrite due to doubts about the legitimacy of their discovery by A.W.G. Kingsbury. Additional uranium species await confirmation through analytical testing, as initial quantitative analysis suggests a combination of elements not fully consistent with the listed species.