This is an interesting thread.
According to Wikipedia, The U.S. Congress passed legislation that would allow U.S. citizens to take possession of unoccupied islands containing saltpetre. http://en.wikipedia.org/wiki/Guano_Islands_Act http://upload.wikimedia.org/wikipedia/commons/0/00/Chincha_guano_islands.JPG Note this is supposedly how we obtained the famous Midway Island. There is lots of information info on the web about salt-petre and caves. For example: http://www.dmme.virginia.gov/DMR3/dmrpdfs/vamin/VAMIN_VOL47_NO04.pdf
