> Sweden’s SSAB, for example, is building an underground cavern, where it will store hydrogen it has created using off-peak North Sea offshore wind
I found the following information about the SSAB pilot cavern:
The pilot plant has a size of 100 cubic meters. At a later stage, a full-scale hydrogen gas storage facility measuring 100,000 to 120,000 cubic meters may be required which is sufficient to supply a full-sized sponge iron factory for three to four days.
Even then. Hydrogen quickly moves upwards. If it is a ground storage, any burning should happen above the storage and not enter it (at least in theory). And you can measure changes in pressure quite easily. I mean, it is not like hydrogen storage has not been done for more than 100 years.
I found the following information about the SSAB pilot cavern: