On Big Island...The Great Crack.
Also known as the Hilina Slump...the crack is roughly 5,000 miles in length. The slump is caused by the movement of the Pacific Plate as it moves and erosive behavior of a debris avalanche (or underwater landslide)
The widening (movement) thus far appears to be only about 10 centimetres a year. An undersea formation of a "buttress" may actually prevent future catastrophic detachment.
Since the Crack hasn't widened and a great portion of Big Island swallowed up by the Pacific...and although the crack isn't particularly scenic...there are still those who "Hike the Great Crack", perhaps because it is there.
https://youtu.be/eLfv0z0DEJE
At some points the crack is very small...at some points sixty feet wide and just as deep. Its scope is very difficult to capture in photographs from the ground.
A better view and some NASA photographs in this article
http://www.instanthawaii.com/cgi-bin/hi?Hikes.crack
On the west side of Oahu is the Ka'ena Point Trail. Running alongside the Pacific.
The trail is an easy 9.5 mile (less than 100 feet elevations) hike along the North Shore. The dirt road used to be an old railroad bed.
During whale season you may spot humpbacks.
Monk Seals
Or occasionally Green sea turtles.
And although the lighthouse at the point is much less than impressive these days
It is little more than an abandoned base with graffiti and a mirror light.
The over-all journey is worth the hike.
Ka'lena Hike
https://youtu.be/sKsuqUgxVJw
And the Petroglyphs on Maui.
Pu'ui Kilea on Maui.
Known as the Olowalu Petroglyphs it remains one of the most easily accessible petroglyph sites in the Hawaiian Islands. There are about 100 images carved here by the ancient inhabitants of these islands.
And the trek to see the Olowalu Petroglyphs.
https://youtu.be/y9E3w6lK8MM
and Olowalu Valley Hike
https://youtu.be/rE-S9N4U26w
More anon...