We first addressed this question almost a year ago (July 2007, to be precise), and were intrigued by it ourselves, since we happen to drive past them every day on the way to the office and back. Here's the lowdown on those holes.
Wells Cargo is owned by a Nevada family and has been around for 80 years or so. They started out in the trucking business -- hence Wells Cargo -- then got into construction. The pits you observed have been and continue to be excavated for gravel for building projects and our friendly source estimated they're about 200-300 feet deep and yup, probably a couple of blocks wide (well, however far the distance between Spring Mountain almost to Flamingo is on one side, and Spring Mountain to Desert Inn on the other).
Currently, the one on the north side is still being excavated while the other one is now being filled (they're both registered non-organic landfills, so they act as a general dumping ground for any construction project that has a bunch of old concrete or excavated foundation spoils that they need to get rid of). This came as something of a relief to us, as we'd noted the holes apparently getting bigger and bigger over the past couple of years and, as they are indeed located right in the middle of a residential area, we wondered if this was safe or if we might just slide into the abyss one rainy night. Guess not.
If you have no idea what we're talking about, click here for a good aerial view.
