The 3.5 km crater is nearly a perfect circle and is a relatively recent 1.4 million years old.
A few months back, the NASA’s Mars Reconnaissance Orbiter returned an amazing picture of a seemingly bottomless pit about 100 meters in diameter. The photo shows no evidence of a wall, and there is speculation that it may be an entrance to an underground cavern.
Strange Surface Feature on Mars
A subsequent photo has shown a vertical wall along one side, so a minimum depth can be established, but there is still no way to tell how deep it actually is. Several other similar holes have been found, but the one above is the most striking because of its symmetry.
Here is something more approachable.
“Rowan Leaves and Hole” by Andy Goldsworthy
This is a bit more down to Earth and of a human scale…