Water showers down heavily from the 10-foot ceiling of the tunnel, and each splash echoes. On the floor the water is ankle-deep.
This dark passage — our only illumination is from hand-cranked flashlights — is part of a tunnel and pipe system that supplies water to 40 percent of Windward Oahu residents from Kahaluu to Kailua. Here, 1,500 feet inside the Koolau mountains, we are about as close as anyone can get to what is arguably Oahu’s most valuable natural resource.
When school groups take the tour, the students are given cups, said our tour guide, Board of Water Supply community relations specialist Arthur Aiu. That way they can taste clear, fresh, pure water straight from its source.
The entrance to Waihee Tunnel is about a mile after the dead end of Waihee Road in Kahaluu Valley.
Once inside the 59-year-old porous shaft, it is bright and dry one moment and dark and wet the next.
The Waihee dike tunnel can yield as much as 7 million to 9 million gallons of water per day. But to keep from depleting its source, the Board of Water Supply limits the yield to about 5 million gallons per day.
The left branch of the Y-shaped tunnel leads to what is called the wet bulkhead, while the right leads to the dry bulkhead.
The dry side is dry only in a relative sense. The water on the floor is still ankle-deep, with occasional drips from the ceiling.
At the end of the tunnel is the concrete bulkhead. That prevents leakage from the porous tunnel, so water collects and builds up over time, serving as what Aiu calls a "bank account."
Water is stored in winter for the dry summer months.
The wet bulkhead lives up to its name. Soon my clothes are soaked.
Rainwater from the ridge of the mountain, 2,000 feet above, is filtered for nine months before it reaches the tunnel and pipe system.
The flow of water from Waihee Tunnel is powered by gravity, so in a power outage, Waihee continues to supply water to residents.
Across Oahu each day, about 150 million gallons of water is pumped to homes, a figure Aiu said has remained fairly constant in his 16 years with the agency. He said he believes the relatively steady rate is due to Oahu residents becoming more aware of water conservation and taking water-saving measures at home.
The rest of Windward Oahu residents receive water from smaller wells located in Kaneohe or Punaluu, Aiu said.
The agency also has dike tunnel systems at Waimanalo, Luluku, Haiku, Kahaluu, Palolo, Manoa, and two Waianae tunnels, Kunesh and Waianae Plantation. But the Waihee tunnel is the only system with a bulkhead storage compartment.