Richie
The "sunken room" is most likely a babai pit, and it strikes me odd that Tukabu would not have recognized it as such. There are several on Niku, and they really are not quite large enough to "go into" other than walk through if you wanted. As described below, they are only a couple of yards deep - maybe a yard at most on Niku - and perhaps up to 12 yards long.
google babai pit and you will find many hits that say things like:
"The coconut economy of such coral islands as those of the Kiribati Group is a difficult one, demanding the utmost of the inhabitants. The idyllic life depends, not on the beneficence of Nature, but upon the resources and energies of the people. Here the human has had to develop techniques that permit survival. To grow taro or babai, the starchy root that is one of his main food crops, he must excavate a pit about twelve yards square and perhaps two yards deep - deep enough, anyway, to reach down to the table of fresh water that collects at sea-level below these porous islands where no fresh water flows."
http://www.janeresture.com/kirricom/We've never seen a cat on Niku, other than the skeleton of one in the old village store, but we've seen abundant rats. The rats don't seem to have slowed down the bird population. I think one factor is that the Coco crabs and land crabs eat the rats (and probably cats too). In the rest of Micronesia, most of the Coco crabs have themselves been eaten, but not on Niku, so maybe the natural checks and balances still exist there.
amck
http://pipa.neaq.org/2009/09/searching-for-invasive-species-on.php
Searching for invasive species on Nikumaroro
Today, after a spectacular series of dives, PIPA director Tukabu Teroroko, Tuake Tema, Rob Barrel, Alan Dynner, Kate Madin, Larry Madin, Brian Skerry, Jeff Wildermuth and I landed on Nikumaroro to check for the presence of invasive species.
Invasive species are organisms that do not belong there and were brought by humans. Nikumaroro is uninhabited today, but over the centuries there had been periodic settlements. We were checking for rats, cats, rabbits, and other organisms that can harm the native animals and plants. Kiribati has successfully worked hard on Phoenix and McKean Islands to eradicate rats and rabbits. But Tukabu and I wanted to check for rats here on Nikumaroro. He knew there were cats on this island, but rats are more devastating to the hundreds of thousands of birds that call Nikumaroro home, and if he found strong evidence, he would plan an eradication.
We explored a small portion of the island, found old village structures, including a sunken room with coral walls that Tukabu said was perhaps an ancient marea, a place of worsiop. Tukabu looked at the fallen coconut fruits and reasoned that rats might not be too bad here, as they were not abundant bite marks on the coconut. Part of the long-term management plan for PIPA is to eradicate invasive species on all the islands to protect the amazing bird life on these islands; the Phoenix Islands are considered among the most important seabird nesting sites in the Pacific.