This is an oldie, but a goodie. For some reason I was asked about it 3 times in a span of a week. Suppose your system was not shut down cleanly and it refuses to come up. During bootup fsck refuses to run, complaining about corrupt superblock. So, what do you do? First, get the alternate superblock locations and then run fsck using one of the alternate superblocks.

To get alternate superblock locations do:

newfs -Nv /dev/rdsk/c0t0d0s3

This will not actually format the slice. It will only print what it would do, if it was actually formating the slice. Basically, it is newfs dry run. It will also give you locations of alternate superblocks on the slice. I guess it should be mentioned that if you used non-default values while formating the slice originally, and you did not make a note of those values, this might not give you valid superblock numbers.

After you have your alternate superblock numbers you can use them to help out fsck (in this case superblock I am using is in block 8192):

fsck -y -F ufs -o b=8192 /dev/rdsk/c0t0d0s3

It might not hurt to run the above with -n option instead of -y to make sure it produces the results you are looking for.

Hopefully, the filesystem is not hosed so badly that it can not be repaired. This was more of an issue when Solaris did not have ufs logging. Nevertheless, it does not hurt to know this for one of those emergencies.