Hmm, snake dens or snake nests?
I've heard the term "den of snakes," aka, "den of thieves." But Google provides no links to "snake dens," just "snake nests". So once more, I believe our alert Dave is correct.
I'm glad that I have no intimate knowledge of snakes. Well, there have been a couple of experiences, now that I recall. Here is one which changed me:
We were living on the edge of Albuquerque, and I was a young mother out walking in the nearby mesa with my two-year-old and a newborn in a tummy pouch. There was an archeological dig nearby, and I was interested to see what the many workers were finding. I wouldn't get too near, but I was amazed at their alacrity. I continued on, walking now on the edge of a canyon that dropped into what had been a river bed. As I continued, I passed an older man talking to a younger worker. He stopped me and began scolding me. "You're out here wearing shorts," he said, "aren't you aware that there are snakes out here?" Then he pointed out a rattler coiled near me--and my little guys--just six feet or so away. I would have passed right by it.
I never wear shorts out in the wild anymore. I haven't bought the boots he counseled me to get, but I'm much more careful to look at where I'm going.
Snake dens or nests, it only takes one...