I recall hearing about some research a few years ago into 3D data storage based on quartz crystals. They were able to get an extremely high information density, hundreds of terabytes on an object you can fit in your hand. Also, the medium is extremely durable; you could bury it in the ground and the data would remain perfectly readable for billions of years. The equipment for writing and reading the data (and creating sufficiently precise crystals) is still pretty rare and expensive, but the proof-of-concept suggests that it could make its way into widespread use someday.

https://en.wikipedia.org/wiki/5D_optical_data_storage