The entire south shore of Oahu is ancient coral reef that is highly porous and attached to the sloping volcanoes on Oahu. There are major springs and caves all along Pearl Harbor. HART rail has had huge problems during construction never made public. Ancient Hawaiians made extensive use of these resources for agricultural and religious purposes.

This is why the downtown section of rail construction will be an incredibly expensive construction nightmare - high water table and rising ocean sea level.

Below is one of my articles on this topic: 

http://www.hawaiifreepress.com/Portals/0/Article%20Attachments/Honolulu's%20Karst%20History.pdf

Another to one of many blogs on the topic - the real history of Oahu...

http://ewa-hawaii-karst.blogspot.com/2015/09/ewa-huge-karst-sea-caves.html

Peter Young's blog also provides some specific history on the underground caves and streams on Oahu...

http://totakeresponsibility.blogspot.com/2012/12/moiliili-karst-moiliili-water-cave.html