Glacier is just run on S3 with some sleep statements added.

I recall at launch just about the only implementation detail that _was_ publicly given was that it did not involve tape. That's going to be difficult to dig up a cite on years later.

No idea how it's evolved over the years, so for all I know it's tape based these days.

I’m sure it’s a mix. Back when it launched there were a number of rumours about it being Blu-Ray based. They had similar capacity for the space used compared to tapes, were considered very physically stable storage mediums, but had long access time as they would need to be physically moved, like tape, explaining the retrieval times.