Bone-Eater's Well

is a well found in the shrine that Kagome Higurashi lives at in the present day it is hidden inside one of the buildings of the shrine which is usually kept locked up. In the Feudal Era it is located in a small forest clearing; the well is used to dispose of bodies of demons and monsters but it also has the purpose of being used as a gateway between the present day and the Feudal Era.

Part in the story
The well is just like many usual wells in the Feudal Era, except that the well possessed an ability to connect the present era: the Heisei and the Reiwa eras that Kagome comes from with the Feudal Era. How the well is active is currently unknown, but several aspects of the well can be deciphered based on observations made throughout the story: It can be presumed that only certain beings may pass through the well; a number of objects have the ability to as well.
 * The only human that is able to travel through the well is Kagome.
 * Inuyasha can travel through without any need of the Shikon no Tama; it is mostly because of his connection to Kagome that allows him to move between the two eras of time.
 * Neither Sōta nor Shippō are able to pass through the well; however, Sōta's voice could reach Inuyasha for unknown reasons while Shippō had no success what so ever.
 * Mistress Centipede was killed and her bones disposed of in the well. She then was revived because she was near the Shikon no Tama (which was still in Kagome's body, unbeknownst to her) so it can be assumed that her bones didn't travel to the future and instead sat at the bottom of the well for 500 years until stimulated by the jewel.
 * Yura was able to send her hair to wreak havoc in the future; most likely because of the jewel shard she stole.
 * Both Sō'unga and Saya were able to pass through without the need of the jewel.

Uses
The Bone-Eater's Well is a mystical well used initially to dispose of the bodies of demons and monsters, whom proved near unbeatable (that is, Mistress Centipede due to her constant resurrection). The Bone-Eater's Well is a major aspect in the series due to its ability to act as a portal between the past and the future. This is obviously invaluable to Kagome who transits via the Bone-Eater's Well to go to the Feudal Era and back, (as well as Inuyasha). Although other demons can also travel through the well too, such as Mistress Centipede.

Known users of the well
The demon's bones were sent to the future by leaving them in a well for fifty years before the story start by Kikyō and her followers. She was able to resurrect herself after being dead and in a well for 500 years, thanks to the Shikon no Tama and then returns to the past via the same well once fully resurrected. Sō'unga was perhaps the very first object that traveled through the Bone-Eater's Well. The sword was sent to the future along with Saya.
 * Mistress Centipede (once revived)
 * Kagome Higurashi
 * Inuyasha
 * Sō'unga

Trivia

 * During both manga and anime, Kagome can go between both eras without the jewels' power. However, she needed it after Inuyasha temporarily forced her to go back home in "Go Back to Your Own Time, Kagome!".
 * Shippō had the jewel's power and is a demon. However, he didn't go through the well.
 * Anything taken by Kagome and Inuyasha when they jump into the well comes out with them on the other side, such as Kagome's bike and bow from Mount Azusa.
 * In InuYasha the Movie: Affections Touching Across Time, the well gets severely damaged when a tree grows through it after Menōmaru uses the Tree of Ages to absorb all the souls in the region. It is seen getting repaired by Inuyasha and his friends in the credits.
 * In the final episode of the series, Kagome returns to the Feudal Era permanently and, although it is not directly explained, it can be inferred from the disappearance of the Jewel and the embrace of farewell with her mother, that Kagome can never return to the Modern Era. Further confirmation that she can no longer travel through time via the well is in the special chapter Since Then, situated six months after the end of the series.