The bubble cave is the primary focal point of diving in 5 Caves / 5 Graves / Makena Landing.
When diving in Makena, the first stop is the sand cavern where you will likely see a group of small reef sharks taking a rest. A few minutes past the cavern, you will find the bubble cave, which is a cool spot where you can swim in and actually surface in an air-pocket in the lava tube.
If you're lucky, you'll find a monk seal hanging out and will likely find turtles lining the edges of the cavern. It's a tad dark inside the lava tube, so always bring a light, or if your diving with me I'll be sure to have one charged up for you.