Apparently, there are very wealthy individuals trying to secure their Bitcoin via underground, physically secured vaults.
I understand the urge to hide valuables in the physical world. Virtual material however can also be hidden in the virtual world. When hidden in (or derived from) public material whose availability and integrity is already being carefully assured for other reasons, you get to leverage all of that integrity and availability, for free.
Of course, you should always assume your data communications are being monitored when you are accessing relevant online material. The techniques you employ must account for that possibility.