Answer: A Jar of Acid
During the occupation of Denmark by the Nazi regime in the 1940s, Hungarian chemist George de Hevesy came up with a very clever way of protecting the Nobel Prizes of his friends and fellow researchers Max von Laue and James Franck.
De Hevesy knew that there was a prohibition against the exportation of precious metals from Denmark and that if either of the men attempted to take their medals with them when leaving the country, they could be prosecuted and imprisoned or brought to harm.
In order to hide the gold medals in such a way that the Nazis would never find them, de Hevesy dissolved the two medals in nitro-hydrochloric acid (a very powerful acid capable of dissolving gold and platinum). He placed the resulting solution on a shelf with other chemicals in his laboratory at the Niels Bohr Institute where it looked innocuous enough, hardly valuable, and not the kind of solution anyone would wish to toy with.
When he returned to the Institute after the war, he found the solution entirely undisturbed. After precipitating the solution to extract a pile of gold powder, he brought the gold to the Nobel Society where they recast the medals using the original gold that had been so carefully and cleverly hidden by de Hevesy.