Couldn't we just hollow out mountains to form very tall, deep lakes, and then pump water in and out to use as a battery? Say, a water cylinder 200m wide and 1000m tall holds approximately 43GWh of potential energy.

So they want to build a hollow structure, 1 km high, with all its weight concentrated at the very top (when charged)? How is that supposed to not immediately collapse?

It is possible if the weights are not too heavy. Does it make economic sense? Probably not.

It probably makes economic sense considering the marketing.

it's not a hollow structure, it's space set aside in an otherwise normal commercial/residential tower. like extra banks of elevators. even if it weren't though, the weight of the building would far outweigh the weight of the blocks.

what a silly idea. Never gonna be economical.

Pump water uphill for gravity energy storage. Dont use traincars or elevators or human-built elevated structures.