What’s the pricing compared to stored energy like pumping water up a hill? I get that it’s not exactly a universal strategy but not like underground caverns don’t have the same issue
The main issue with pumped storage is you need a lot of water. Imagine a elevated water tank like that used on farms. 20,000 gallons (75,000 litres) of water elevated at 18 feet (5.4m) has 1.1kWh of potential energy.
900m is a huge elevation difference. The highest pumped power dam currently in Japan (by hydraulic height, after looking at almost all the Japanese hydro pages on English Wikipedia) is about 780 m; the median is just under 400 m. Presumably most of the other hydroelectric dams have already been investigated for pumped power, but lack a sufficiently large lower reservoir.