Download and unpack PETSc's source distribution:
Browser download:
ftp://ftp.mcs.anl.gov/pub/petsc/software_old/v2.1.6.petsc.tar.gz
Unpack:
tar -x -z -f v2.1.6.petsc.tar.gz
creates the directory
petsc- where
x.x.xx.x.x is the PETSc version.