'Partnering with the planetary mobility firm Lunar Outpost, the California-based social intelligence company LunarCrush plans to deliver a treasure chest of 62 Bitcoins currently worth about $1.5 million to the surface of the moon.

"The Bitcoin bounty, dubbed Nakamoto_1, will be unlocked by the first space traveler to reach the moon and obtain the wallet's private key, which will be etched onto a Lunar Outpost MAPP rover launching into space later this year," a press release about the project states.'

The top secret key will be recorded, sent to the rover construction site, imprinted and attached to the vehicle. It will be big enough to be read by someone wearing a space helmet.
Probably 38 people will know the hidden private key to $1.5mill.