Google Maps

[Midnight Rambler]

If you're talking about the thing to the left of where the marker puts you, mauka of the prison buildings, it's a water catchment. The trapezoidal area is a paved surface that's the actual catchment, and right below that is a rectangular reservoir. At Puu Waawaa, just above and left of where this link takes you, is a similar but much more extensive one (now in disrepair).
https://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=Puu+Waawaa+State+Wildlife+Sanctuary,+Kailua-Kona,+HI&aq=1&oq=Puu+Waa&sll=19.73433,-155.890385