yup! More fun facts about a 1 km head height... Off-the-shelf turbines are actually spec'd for a max head height of something closer to .5 km. So the design calls for a double-drop. This design approach is taken from the DOE research linked on our website.

so where the geology allows it, why not go even deeper with the lowest reservoir, and put multiple turbines in series, with perhaps small reservoirs each .5 km?

Then the total energy capacity is V * rho * g * h, so that energy store is proportional to height, while tunnel boring price is roughly constant as long as tunnel boring volume of the reservoirs is much larger than the volume of vertical shafts.

I realize its a bit oversimplified but if we consider 2 prices: p1 price per volume for boring horizontally (for reservoirs), and p2 price per volume for boring vertically, then increasing the reservoir size by a volume delta V, requires boring 2 * delta V (upper and lower reservoir), while boring vertically the difference in height depends on the diameter...