Invasion Caches
Based on what records exist, there were one thousand of these built in the USA in 1954 and 1955: twenty to a state, including what is now Alaska and Hawaii (ironically, the most well-preserved Invasion Cache publicly found to date was located outside Anchorage, Alaska). However, no Invasion Caches have been found in Puerto Rico, or any of the USA’s other overseas territories. Presumably, the government agency who built these decided to make the reasonable assumption that Alaska and Hawaii would be states before too much longer.