The Mars rovers are designed for off-roading, mostly because the entirety of Mars is off-road. But, they're driven very conservatively, since if they ever do get stuck, well... Yeah, that's kind of the end of them. Problem is, it's hard to explore (or take samples) when you can't get into all of those interesting nooks and crannies, and one solution that the European Space Agency is exploring is to just send two robots: a traditional rover, and a deployable tethered snake.
The concept here is that you'd have a wheeled rover capable of moving efficiently over long distances, with a snake robot attached to it somehow. The "somehow" part is still in the works, but there are some interesting options:
One option is to make the robot into one of the vehicle's arms, with the ability to disconnect and reconnect itself, so that it can be lowered to the ground, where it can crawl about independently. The researchers envisage using the rover to navigate over large distances, after which the snake robot can detach itself and crawl into tight, inaccessible areas. A cable will connect the robot to the vehicle. The cable will supply power and tractive power, i.e. it can be winched back to the rover. Communication between the pair will be facilitated via signals transmitted down the cable.
Another scenario illustrating how the vehicle and the snake robot can work together is for the robot to be located underneath or on top of the rover. That would require a hoisting mechanism to pick up the robot, lift it up and connect it to the rover.
A feasibility study is being conducted by SINTEF ICT in Norway, and the snake robot that they're using appears to be this one, called Wheeko:
This video is several years old, but in any case, this isn't the robot snake that will be going to Mars. The current study is just trying to see if the basic concept will work, and if everything looks good, a proposal for a more specific and detailed study will be submitted by the end of the year.
Illustration: SINTEF ICT