At the north end of 2-mile wide Kihilo Bay (on the South Kohala Coast), Blue (Wainanali'i) Lagoon is a luminescent streak of turquoise that lures hikers from a scenic viewpoint on the highway. Many walk an unmarked trail from near the turnout—largely because one rental car tends to attract another, and people figure it's the thing to do. The round-trip hike is a little over 2 miles, dropping 200 feet through an arid kiawe tree forest that provides ample opportunity to take a wrong turn.
A second route to Blue Lagoon follows palms trees along a black-sand beach past a couple luxury homes and historic cottages, and involves no climbing. You begin by taking a state park road that is about a quarter mile from the scenic viewpoint. This option includes a visit to a fresh water pool used by ancient Hawaiians.
Blue Lagoon is turtle city. Many of the creatures haul out here, turning salty-white on the rocky shoreline, and then taking a slow dip in the water, which turns their shells a glistening amber-green. The "lagoon" is actually part of a massive fishpond built by Kamehameha the Great in 1810 and considered and engineering feat by early Western visitors.
Swimming here is a novelty, but the snorkeling is not among the Big Island's best: underground fresh water intrusion makes it chilly and a silty bottom can make visibility only fair. See Hawaii the Big Island Trailblazer page 65 for more details.