The underworld itself—commonly referred to as Hades, after its patron god, but also known by various metonyms—is described as being located at the periphery of the earth, either associated with the outer limits of the ocean (i.e., Oceanus, again also a god) or beneath the earth.[3][4] Darkness and a lack of sunlight are common features associated with the underworld[5][6] and, in this way, provide a direct contrast to both the 'normality' of the land of the living (where the sun shines) and also with the brightness associated with Mount Olympus (the realm of the gods).[7][8] The underworld is also considered to be an invisible realm,[9] which is understood both in relation to the permanent state of darkness but also a potential etymological link with Hades as the 'unseen place'.[10] The underworld is made solely for the dead and so mortals do not enter it – with only a few heroic exceptions (who undertook a mythical catabasis: Heracles, Theseus, Orpheus, possibly also Odysseus, and in later Roman depictions Aeneas).