Complex sentence: The Coral Sea Islands were first charted in 1803; in the 1870 and 1880s the islands were mined for guano but the absence of a permanent supply of fresh water prevented long-term habitation.
Split into simpler sentences: The Coral Sea Islands were first charted in 1803. In the 1870s and 1880s the islands were mined for guano but the absence of a permanent supply of fresh water prevented long-term habitation.