One of the old sugar plantations had dug a series of canals for irrigating the crops. A large portion of these canals were actually tunnels through the mountainside. Now that the sugar plantation is no more, Kauai Backcountry Adventures is offering hour long tube rides through the canals. You spend about 70% of the time in the pitch black tunnels. You wear a helmet and a headlamp to see where you where you are going. This is an incredibly fun activity that I cannot recommend enough!