Answer: In a strand of DNA.
Scientists have discovered that almost any file can be stored in synthetic DNA strands, from Word documents to whole operating systems. Startup company Carverr has decided to leverage this technology to allow customers to securely store the information for their cryptocurrency wallet, CNET reports.
Using an encrypted email service, ProtonMail, customers send their private cryptocurrency key to Carverr (for extra security, they recommend sending an encrypted version). The company will send back a couple of small vials containing the DNA, which should be stored in the freezer away from the light.
The idea is that this method of securing data will survive the test of time and innovation much, much longer than any hard drive. As long as there are labs and scientists around, the data contained within the DNA will be retrievable.