IT is possible, even without using a professional data recovery company. In order to make sure the data is not readable you have to write over the original sections, but think of it this way, If you've ever recorded over an old cassette tape or VHS video, you can see/hear some of the old songs/video images on the tape, same works for hard drives (to a lesser extent) the bits on the drive still maintain some of their old polarity (+ or -) which can be readable. You need to "write zeros" to your hard drive, in other words neutralise the polarity of each bit. But even this is only 99% accurate. A friend of mine works with a data storage company that specialises in restoration and destruction of hard drives. And the only surefire way to get rid of it, with no data recoverable, is to shred the plates, which is what we had to do last year to some hard drives. If you need to recover stuff you can do it yourself, just look for some stuff around the net. Not sure what progs are good nowadays, used to have a few good ones a few years ago, but you can get 90% of the data back after a format.
Yes, it can be done, if data has not been over-written. Numerous forensic tools, which professionals use, can retrieve this data.
Even if it is over-written, there is a chance that you could get some data back.
Truly the only way to get rid of the data is to over-write it, or magnetize it, as suggested, or even completely destroy the drive physically. This is why there are wiping programs that over-write with patterns of 0s and 1s for secure erasing.