Tales of heroes traveling to the Land of the Dead pop up in some form or another in every culture's mythology. This convention is so widespread that it even has its own fancy name: katabasis. So, Theseus and Pirithious are definitely part of a much larger tradition when they head down to the realm of Hades. Although, we have to point out that most of the time the heroes' intentions are a lot more pure than Theseus and Pirithous's, who want to kidnap Persephone, queen of the Dead.
There's a bunch of other examples of the katabasis even in Greek mythology: Orpheus heads down in a failed attempt to rescue his lover Eurydice; Psyche journeys there to regain the love of Eros (Cupid); Odysseus travels there on his quest to return home; Persephone, herself, travels back and forth every year. Also, we can't forget Heracles who travels there to capture the hellhound Cereberus and ends up rescuing Theseus while he's at it.
The oldest example of a hero journeying to the Underworld happens in the Epic of Gilgamesh, which is thought to be the world's oldest surviving piece of literature. In this story, the warrior-king Gilgamesh travels to Land of the Dead to learn how to be immortal from a dude named Utnapishtim. Just as Theseus and Pirithous fail in their quest to kidnap Persephone, wife of Hades, Gilgamesh fails to in his quest and never learns how to live forever. Just like most everybody else, Gilgamesh eventually relocates to the Land of the Dead for good.
The Christian religion is totally inspired by a journey to death and back. The resurrection of Jesus after being dead for three days is one of the cornerstones of the entire faith. Some Christians take the idea even further and believe the Christ literally went to the Land of the Dead. This journey is sometimes called the "harrowing of hell." Different branches of Christianity have different versions of this story.
Some Christians say he went to Hell to shame the souls being punished there, while others say that he went to Limbo to inspire those that hadn't quite made it to Heaven--there are tons of variations. No matter which way you shake it, though, there are about 2.1 billion people in the world who believe that a journey and return to the Land of the Dead is completely possible.