It might also be connected to a metal pipe or another ground rod inside.
2018 IRC
E3610.2 Securing and protection against physical damage.
Where exposed, a grounding electrode conductor or its
enclosure shall be securely fastened to the surface on which it
is carried. Grounding electrode conductors shall be permitted
to be installed on or through framing members. A 6 AWG or
larger copper or aluminum grounding electrode conductor not
exposed to physical damage shall be permitted to be run
along the surface of the building construction without metal
covering or protection. A 6 AWG or larger copper or aluminum
grounding electrode exposed to physical damage shall
be in rigid metal conduit, intermediate metal conduit, rigid
polyvinyl chloride (PVC), nonmetallic conduit, reinforced
thermosetting resin (RTRC-XW) nonmetallic conduit, electrical
metallic tubing or cable armor. Grounding electrode
conductors smaller than 6 AWG shall be in rigid metal conduit,
intermediate metal conduit, rigid polyvinyl chloride
(PVC) nonmetallic conduit, reinforced thermoseting resin
(RTRC-XW) nonmetallic conduit, electrical metallic tubing
or cable armor. Grounding electrode conductors and grounding
electrode bonding jumpers in contact with the earth shall
not be required to comply with Section E3803, but shall be
buried or otherwise protected if subject to physical damage.
[250.64(B)]
Bare aluminum or copper-clad aluminum grounding electrode
conductors shall not be used where in direct contact
with masonry or the earth or where subject to corrosive conditions.
Where used outside, aluminum or copper-clad aluminum
grounding electrode conductors shall not be installed
within 18 inches (457 mm) of the earth. [250.64(A)]