Midoriko's cave

Midoriko's cave, commonly known as the, is cave where the Shikon Jewel was created after Midoriko, the all power priestess who created the Shikon No Tama defeated the demons. The cave is situated near the home of demon slayers and Sango guides Inuyasha and the others to the cave and explains the story behind the jewel.

Trivia

 * The entrance of the cave is protected by a powerful spiritual barrier, which was believed to be created by Midoriko herself, and even within the Shikon No Tama she was able to maintain the barrier.
 * It is also shown that no one can pass the barrier, unless the one who wishes to enter sympathises with Midoriko, who regerts in creating the Shikon No Tama and the destruction it follows. Also thinking evil inside the cave will cause the person to be immeadiately thrown out of the cave (as what happened to Inuyasha after thinks to use the Shikon No Tama to become a full-fledged demon. And was noted by Shippō as Midoriko's version of "Sit".

Reference
钟乳洞