Since his debut in the '70s, Nightcrawler has used his teleportation ability to aid his fellow X-Men on countless missions. With this power, Nightcrawler is able to disappear in a puff of smoke and then reappear somewhere else seconds later. This, however, brings up a question: where does Nightcrawler go during the brief seconds in which he isn't present?
First appearing in Giant-Size X-Men #1 (written by Len Wein with art by Dave Cockrum), Nightcrawler joined a brand new team of X-Men on a rescue mission to save the original team from the mutant island Krakoa. In this issue, Nightcrawler displayed a unique ability to teleport short distances, leaving behind a thick layer of smoke and a putrid, sulfuric scent. Following the events of this initial appearance, Nightcrawler would join the X-Men full time, becoming a fan favorite in the process. However, the truth about Nightcrawler's teleportation powers wouldn't be revealed until much later in the X-Men: Evolution animated series.
In the episode "Shadowdance," the mutant inventor Forge creates a device that allows Nightcrawler to teleport to locations further than two miles away. While using the device, Nightcrawler briefly passes through a hellish dimension populated by dinosaur-like creatures. Through his continued use of the device, Nightcrawler unintentionally opens a doorway that allows the creatures to enter the X-Men's dimension. By the end of the episode, Forge builds another device that banishes the creatures back to their world, but future creators now had an influential precedent for where Nightcrawler goes when he teleports.
The concept of Nightcrawler passing through another dimension would be expanded upon in Marvel comics during the much-maligned storyline "Draco" by writer Chuck Austen and artist Sean Phillips. In the prelude to the story, it is revealed that Nightcrawler's true father is a demon known as Azazel from a hellish reality known as the Brimstone Dimension. Nightcrawler and the X-Men are transported to this reality, where they are subsequently captured by Azazel. While Nightcrawler is detained, Azazel explains that Nightcrawler and all of his other teleporting children briefly travel through the Brimstone Dimension while using their powers. Azazel then reveals he is trapped in the Brimstone Dimension and intends to use Nightcrawler to open a new gateway for himself and his army.
Before Azazel can enact his plan, Nightcrawler teleports away with his half-brother Nils and attempts to rescue the other X-Men. Magnetic mutant Polaris opens a dimensional rift in Nils' stomach that allows the X-Men to escape back to Earth, but Nightcrawler is left to face his father. Before Azazel can kill Nightcrawler, he is stopped by yet another of his children known as the Black Kiwi. The story cuts back to the X-Mansion, with Nightcrawler talking to Professor Xavier about his bizarre adventure. With this, we finally have an answer to where Nightcrawler goes when he disappears. During the brief moments he's gone, Nightcrawler passes through an alternate reality inspired by that seen in X-Men Evolution. However, since this comes from possibly the most hated run in X-Men history, it's a detail which may be reworked or retconned by future writers of Nightcrawler's adventures.