A six letter inexhaustible resource with the second to last letter a "u"

The inexhaustible resource with the 6 letter and second last of that alphabet be u, is Uranium

Explanation:

Any natural resource which will eventually gets over in the world and there has been no way to make it out by itself or by technology, falls under the category of inexhaustible resources.

The Uranium is the word with 7 letters as the 6 letter name with u at the second last position has no other option. Uranium is a natural resource, it is found deep within the core of Earth and is used in for electricity generation purpose as when in sufficient amount, it explodes by itself or disintegrate giving out energy by nuclear fission.