deep hole on mars

The comment above was pretty close to reality. They use radar, they send a beam then measure the time it took the beam to go down and reflect back to the antena, since the beam is microwave radiation you know it goes with the speed of light c and then it's a matter of simple math to determine the distance If you use an SAR (synthetic aperture radar) you can generate an image and with the TOF (time of flight) data mentioned above you can generate a 3d image or depth map and if you use dopler radar you can also see the speed at wich stuff moves ... yup the same dopler radar used by cops only more sensitive

Sending some kind of waves and measuring them when they bounce back would be my not very educated guess.

I think that there are a lot of things that are well beyond the capability that we as ordinary people would ever be able to understand. That said, I think there are different technologies that we will never be able to understand. My assumption would be that they would be able to estimate these kind of things based on the diameter of the holes compared with the assumed depth based on pictures that are taken by the satellites and such. However, I don't think that there is 100 percent accuracy with any of this.

I have no clue how they go about measuring things of that capacity. I'm sure they have special people for determining that. I read that article this morning, too. Some middle school students and a teacher actually discovered it through a summer program at Arizona State, quite amazing what a group of positive kids can do.