This legendary beach on Mahana Bay isn't really that green, but it is a rare and beautiful sight. Its color comes from crystals of olivine, the mineral found in the semiprecious gemstone known as peridot. Olivine is created in high-heat environments – like during the formation of stars or volcanic eruptions. This batch comes from the latter, and is what's left behind as waves erode the littoral cone looming above the cove. Swimming is only advisable on the very few exceptionally calm days.
To get here take the left (east) fork of South Point Rd some 10 miles from Hwy 11 to the old barracks. Park here (don't leave any valuables in your car) and hike the dusty, windy, hot 2.5 miles to Mahana Bay. Start by heading south to the Kaulana boat ramp then veer left (east) following the coastline. The unceasing winds will sandblast your face the entire way making the trip feel twice as far as it is. Bring lots of water.
Alternatively, you can support the local economy by bouncing along in the back of one of the dilapidated 4WD pickups that usually cluster at the barracks offering rides for cash (one way/round trip $10/15) – an adventure in itself.
Whether you arrive on foot or off-road vehicle, you'll have to scramble down the cliff to the beach, which is becoming a major tourist attraction despite the difficult access. Go early, late or when it's overcast to beat the crowds.