linguistic:paraphrasing, structural:expansion
Drilling

Drilling at Kola SG-3 began on 24 May 1970 using the Uralmash -4E, a serial drilling rig used for drilling oil wells. The rig was slightly modified to be able to reach a 7,000-metre (23,000 ft) depth. In 1974, the new purpose-built Uralmash-15000 drilling rig was installed onsite, named after the new target depth, set at 15,000 metres (49,000 ft).