The North Crater

The North Crater was created when the extraterrestrial life form  Jenova crashed into Gaia thousands of years ago.  Later, it was sealed away in the North Crater by the Ancients after transforming many of them into monsters.  The WEAPONs were sealed inside the crater as well to ensure that if Jenova ever escaped, the WEAPONs would be deployed to eradicate all that do harm to the planet, including Jenova. Later on in Final Fantasy VII, after being thrown into the Lifestream, Sephiroth drifts until he ends up in the North Crater.  It is there where AVALANCHE must confront and defeat Sephiroth.