If you must preserve messages for people in the far future to read, Blu-ray discs and USB sticks are no good. For real long-term storage, you want a DNA time capsule.
Just 1 gram of DNA is theoretically capable of holding 455 exabytes - enough for all the data held by Google, Facebook and every other major tech company, with room to spare. It's also incredibly durable: DNA has been extracted and sequenced from 700,000-year-old horse bones. But conditions have to be right for it to last.
They also tried to mimic the way fossils keep a DNA sequence intact. Excluding all water from the environment was key, so they encapsulated the DNA in microscopic spheres of glass. To test how long this storage system might last, they encoded two venerable documents, totalling 83 kilobytes: the Swiss federal charter from 1291, and the Archimedes Palimpsest, a 10th-century version of ancient Greek texts. DNA versions of these texts were kept at 60, 65 and 70 °C for a week to simulate ageing. They remained readable without any errors (Angewandte Chemie, doi.org/f23gmf).
The results suggest that data in DNA form could last 2000 years if kept at a temperature of around 10 °C. The Global Seed Vault in the Arctic could preserve it for over 2 million years at a chilly -18 °C, offering truly long-term storage.
Grass would like to store all the world's current knowledge for future generations, but it's far too expensive to generate DNA at present. It cost around £1000 to encode the 83 kilobytes, so doing the same with Wikipedia would run to billions. Instead, Grass suggests that we focus on what future historians might want to read. "If you look at how we look at the Middle Ages, it's very influenced by what information has been stored," he says. "It's very important that we get a relatively neutral documentation of our current time and store that."
Sriram Kosuri of the University of California, Los Angeles, thinks the projected preservation times are reasonable. In 2012 he stored a copy of a colleague's book in DNA and is now working with the band OK Go to create a DNA version of their latest album. But he's less sure about what to put in a time capsule. "I haven't given it much thought," he says. "We pretty much chose an arbitrary piece of digital information whose only constraint is that we weren't going to get in trouble for making a lot of copies of it."