Street Artists are known for their ability to create works of art that defy logic by playing with our perception. For many of these sidewalk street art pieces, the artists take advantage of monocular depth cues to create a sense of distance and relative size in their art, thereby adding an illusion of a deep chasm in the picture displayed above.
The artist behind the chasm image played with relative size of the objects in the image to create a sense of depth. By drawing the stalagmites smaller than the stalactites, the artist created the illusion that there is a steep drop down to the base of the chasm.
The artist also used lighting and shadow to add to the illusion. The darker shadows on the water cause the viewer to perceive that the water is far below them, and that the lighter colored stalactites are closer to the viewer.
While we know that this piece of art is really a 2-Dimensional representation of a 3-Dimensional occurrence, the use of monocular cues skews with the viewer's relative perception of the objects within the image to create depth. Street artists take advantage of the monocular depth cues to change their art from a flat drawing to a piece that messes with the perception and minds of the audience.
More examples of street art are available here