A reservoir for containing a fluid includes a cylindrical wall sized to engage a shaft bored into ground, a bottom end cap fixed to a lower end of the cylindrical wall and a top end cap fixed to an upper end of the cylindrical wall. The top end cap is concave such that the top end cap extends below a top end of the cylindrical wall. The bottom end cap, the top end cap, and the cylindrical wall define a subterranean chamber. A plug made of a composite material is on the top end cap, with a lower face disposed on the concave top end cap and a peripheral face engaging the shaft. At least one pressure port extends downwardly into the shaft and through the plug and the top end cap, defining a fluid path to the subterranean chamber.