Gonna ballpark in stupid units to see how wrong my intuition that that’s not very dense is:
Assuming dense but not hot new thing spinning rust, 16TB per 34.5 cubic inch standard 3.5" disc.
(100PB/16TB)*2 (assume at least two spindle redundant) is about 12,000 discs, so about 414,000 cubic inches of just discs without any of the supporting equipment.
A highwall shipping container is like 5,900,000 cubic inches, so only like 7% of that thing would be discs.
Or, accommodating a little bit of the support, let’s say it’s just full of those commodity 90-bay 4U storage servers. Those are 19" x 7" x 26.4" (3511.2 cubic inches) for 1440 raw TB each, again 2 spindle redundancy so you’d need about 140 of the things for 100PB, round up to 500,000 cubic inches of those… still less than 9%.
Yeah, unless I did my math radically wrong, that’s surprisingly not very dense.