GPS is generally the standard to which all other localization technologies are compared, and in most outdoor environments, it's hard to beat for accuracy, precision, and reliability. It's funny, then, that the times you need GPS the most (in places like downtown New York or San Francisco) usually end up being the times that GPS utterly fails due to tall buildings blocking out the sky.
Robots have this same problem, so researchers have been trying to find other ways that a robot can localize itself where GPS is intermittent. A common strategy is to use wheel odometers or inertial measurement units to "guess" where the robot has gotten to since its last external position fix, but accuracy still relies on landmarks to help correct for errors. So let's see, what are some things that are easy to find in urban environments and don't go anywhere? You probably weren't thinking "manhole covers," but yeah, it's manhole covers.
The reason manhole covers might be a good idea is because they have well-defined locations and because they're fairly large and made of metal they're pretty easy for a robot to spot, even if it's dark or rainy or snowy or whatever. With sensitive enough sensors, a robot could detect all the dents and wear that make each cover unique, and fix its position within inches. Sounds great, and it works in tests along with a database of pre-scanned manhole covers, except that generally, you tend to find manhole covers in the middle of the street, implying that a system like this would be best for autonomous cars as opposed to other robots that might not do as well running into traffic to determine their position.
And, yes, that's a picture of a manhole robot. That shoots manholes. Thank you, Mighty Morphin Power Rangers.