In 2008, Indian Strategic Petroleum Reserve Limited started construction of an underground unlined rock cavern complex for storage of imported crude oil at Visakhapatnam for 1.3 MMT (million metric tons) of crude oil. The excavation of the works was finally completed in February 2014. During the construction stage, after 95 % of the total excavation volume had already been excavated, an extreme wedge sliding event occurred in one of the cavern walls in April 2011, which caused a major delay to completion of the project. This wedge slide and other rock mass behaviour encountered during the excavation of the underground storage facility are described and discussed in this paper.