First, let's understand, what is guano. This word, which sounds very consonant with "shit", means the same principle. Guano is the dried excrement of birds and bats.
So, on August 18, 1856, the US federal law was passed, which read as follows:
Translation of the Guano Act
If a citizen of the United States discovers a deposit of guano on any island, reef or rock outside the jurisdiction of any other Government, not occupied by citizens of any other Government, and non-violently takes possession of such, settling on it, such an island, a reef or rock may, at the discretion of the President, be deemed to belong to the United States
Original for Unbelievers
Whenever any citizen of the United States discovers a deposit of guano on any island, rock, or key, not within the lawful jurisdiction of any other Government, and not occupied by the citizens of any other Government, and takes peaceable possession thereof, and occupies the same, such island, rock, or key may, at the discretion of the President, be considered as appertaining to the United States.
Why did the Americans need guano so badly that they decided to collect rocks, reefs and crayons of the island? Everything is very simple. Guano contains a significant proportion of nitrogen and phosphorus compounds, making it an excellent fertilizer. A few centuries earlier, the conquistadors learned about this, who spied how the Indians fertilized their crops with them, getting colossal harvests. By the way, the word itself comes from the Spanish "guano" - which means "litter".
And at the beginning of the 19th century, this was remembered and guano began to be highly valued as an agricultural fertilizer. In 1855, the United States became aware of the presence of rich deposits of guano on the islands of the Pacific Ocean. Congress passed this law to speed up the capture and facilitate control over the sources of the valuable resource.
The law explicitly allowed the Guana Islands to be considered US possession, but also stipulated that the US was not required to retain these possessions after the depletion of the guano deposits. At the same time, the law did not specify the status that these territories will receive after they cease to be of interest to private individuals who have taken possession of them - US citizens.
The law laid the foundation for the concept of the United States' island territories. Previously, any territory acquired by the United States immediately became an integral part of the country, unless its status was changed by an agreement, and in principle had the opportunity to become a state of the Union in the future. The Guano Act declared the emergence of a special kind of territories, which, being under the jurisdiction of the federal government, did not even have the theoretical opportunity to raise their status to the level of the state of the Union. Such territories are also called unincorporated territories.
More than 50 islands have been claimed under this law. Some of them are still under US jurisdiction.