How do you photograph that which can't really be seen? Well in some cases it requires a proxy, something to substitute for and represent the 'thing' itself. Recently while visiting the Point Reyes National Seashore northwest of San Francisco I saw a really nice example of this along the Earthquake Trail. This innocuous looking path takes the visitor right along the famous San Andreas fault line, whose sudden movement resulted in the devastating 1906 San Francisco earthquake. While geological danger might be be directly under ones feet here, you'd never know it just from looking around at the calm meadows and forest through which the path meanders. That is until you spot a series of blue posts stuck in the ground trailing off in a roughly north-south line through the trees. Then you happen upon a few wooden steps between two short lines of fence posts separated by maybe 15-20 feet. Only then, via these proxies do you realize just what you're standing on. The posts of course being the path of the fault line and the two fences representing the site of a fence that was ripped in two by the faults 1906 movement as they lay across the fault.
So just because you can't see the thing itself doesn't mean there isn't a way to represent it in a photo. Just look for a proxy or two.