Lava tube caves are one of the most amazing phenomena in nature. In Kauai, there are several lava tube caves along the Na Pali coast, one of the most stunning coastlines in the world! If you are very fit, you can access them with a kayak tour. Otherwise, you can take a catamaran or an inflatable speed boat tour along the coastline. Blue Dolphin and Captain Andy's run regular tours here. Spouting Horn in Poipu is another great place to see a lava tube cave. Pressure from the ocean water spouts water through an old lava tube.
Here is some more information on how the lava tube caves are formed. Resource: Hawaii Volcanoes National Park.
The formation of lava tubes starts with the formation of basaltic lava. Just like water, lava tends to flow in the low areas of the landscape as it makes its way downslope. The flowing lava is about 2,000 degrees Fahrenheit which is incredibly hot. But when it is on the surface, exposed to the open air, it cools and hardens from the outside in fairly quickly. The lava flow will start to create a channel just like a river. And just like a river in the wintertime, the channel will start to crust or freeze over from the outer edges towards the middle. Once the channel is completely crusted over, temperature loss is minimal over miles of travel. At this early stage in the formation, the tube is wide, shallow and the crust is thin. An increase of lava coming through the tube could cause surface breakouts from the ceiling flowing over the tube system and thickening the crust. Now over time, the lava flowing through the tube will actually thermally erode the floor and down cut, just like a 2,000 degree river. With increased downcutting the tube will actually enlarge and the flow of lava will only possibly be midway, and right above it, will be a section of super-heated gas and air. The intense heat of the lave inside the tube will actually remelt the walls and the ceiling creating soda straw stalactites and other unique formations only found in lava tubes. These delicate structures are only formed in a tube that has an active flow. Now after the eruption stops, the tube system will actually drain out fairly quickly, leaving us with a truly amazing creation of nature that will soon develop into an ecosystem of its own.