On the island of Yap in Micronesia there are hundreds of stone discs, some as tall as a person, that have been used as money for centuries. They were quarried from other islands and brought back (proof of work) and since they were so heavy, when an exchange was made people would simply agree who the owner was without having to move them, and with no written record. Even if one fell off a boat and sank to the bottom of the ocean, it could still be used since its location was understood by the people.