After the Japanese attack on Pearl Harbor in 1941, war planners recognized a special threat: If Japan invaded Hawaii, it could seize millions of dollars in U.S. currency from the islands’ people and businesses — currency that would remain valuable since it was indistinguishable from that on the mainland.

They came up with a novel solution: In January 1942 the government recalled all regular paper money on the islands, except for an allowance of $200 per individual and $500 per business. Then it issued new notes stamped with the word HAWAII. Now if Hawaii fell to Japan these “overprinted” notes could be declared worthless.

After the new notes were distributed, citizens were asked to trade in their remaining regular currency. Two hundred million dollars was burned, and between August 1942 and October 1944 Hawaiians were required to conduct their business using the overprinted notes.

Happily, the invasion never came, and after the war the Hawaiian notes were exchanged for regular currency again. Today they’re collectors’ items.