Nothing quite spoils a family vacation like having a wave sweep someone away and into the depths of the deep blue sea. So, first rule when visiting the very cool Nakalele Blowhole and Bellstone Pools is to stay well back from the reef edges and watch wave action for at least 15 minutes. High surf can be present during calm days. If the reef around the pools is wet, take that as a clue that sporadic surf is jumping the reef and stay out.
On calm days the pools (known as 'Olivine Pools' to some), are a world-class freebie. This photo shows one of a half-dozen soaking tubs, spread out over a couple of reefy acres. Colorful fishies and sea flora lie beneath clear water.
It's a tough, steep hike down to the pool (the price of admission) from an obscure turnout on Maui's north shore, near the ancient Bellstone—a hog-sized rock than has rested here for centuries. When struck at the right place with the correct object, the rock emanates a hollow, metallic sound.
Several miles up scenic Highway 30, the Nakalele Blowhole is a well-known roadside attraction, and rental car clusters normally mark each of the two trailheads. The rough trail down drops about 200 feet over .75-mile. Most people stand farther back than these visitors, but they thankfully have the sense not to have the ocean at their backs—the double whammy.
Again, these folks weren't in any real danger, but you can see how someone who stood closer yet could be knocked down and then floated back into the blowhole—gulp—which is an opening in the ceiling of an underwater cave in the reef. Multi-colored, weirdly formed rocks surround the margins of the reef, nice perches to sit and catch a little spray on hot days.
Maui Trailblazer has more details on these places (pages 74-79), and others on this rural coastline.