The ''Big 3'' islands: Subi, Mischief and Fiery Cross Reef have undergone major construction of military and dual-use infrastructures from 2015 to 2017. Recent Deimos-2 imagery shows the construction of new facilities are still going on or being completed.
The ''Big 3”—Subi, Mischief, and Fiery Cross Reefs located in South China Sea
South China Sea disputed islands
17 March 2015: dredging and construction activities are clearly visible.
23 March 2015: the airstrip starts to be built up along with the point defense in the left corner and the construction site where radar/sensor arrays will be placed.
24 November 2015: the central part of the island is fully covered by lots of new constructions.
24 April 2017: after 2 years of construction works, the facilities in the island include: an airstrip, radar/sensor arrays, hangars, mobile missile shelters and point defenses.
6 February 2018: large underground structures likely intended to house munitions and other essential materiel have been completed and entirely buried.
The Asia Maritime Transparency Initiative gives you the full picture over the situation on the South China Sea.