Arachnaio is the tallest mountain of the county/region. It mainly covers the central part of the region (reaching from Epidaurus to the Argolic flatlands). It also borders mount Didimo to the south. Its highest peak (“Papakostas”) reaches an altitude of 1.199m. A village of the same name, Arachanaio (formerly known as “Heli”) can be found between this peak and that of “Trapezonas” (at 1,139m above sea level). It boasts important sights such as the New and Old monasteries of Talantios. Other notable peaks are “Profitis ilias” (1.180m), “Malie Liseze” in Arna (1.140m), “Siouri” (938m), “Elafokorfi” (meaning “Deer peak”) or “Dreri” (931m), and “Maliza” (857m) which houses a Nature Reserve. Because of the peculiar view that the peaks present when seen from afar, residents of Poros and Troizinia also call Arachnaio “Kimomeni” (meaning “sleeping lady”).

There are many elements that prove the region was known since ancient times. Firstly, the ancient Theater of Epidaurus is built in the mountain's foothills. Pausanias mentions that the town of Lissa was built there, and also that on the top of the peak of “Profitis Ilias”, there were altars devoted to Zeus and Hera, upon which the residents offered sacrifices in times of draught. Also, according to ancient texts (i.e. “Agamemnon” by Aeschylus) Arachnaio was also used as a Phryctoria, which was a system of communications used in ancient Greece, utilizing torches at night. A reason for this is that from its peak, one can see all of the Argolic flatlands. One can also find the ruins of Gyclos Fort on top of mount Arachnaio (in the Skoundi-Arachnaio gap).

On the grounds of its north side, one can see Pinetree forests, but its south side has been completely deforested. A Wind Farm has also been created in the area, due to its high wind potential. Unfortunately, human interaction has brought severe changes for the worse in the region's biodiversity of its natural flora and fauna.