Here's why:
* "Lie" is the correct verb when referring to a state of rest. "Lays" is the third-person singular present tense of "lay," which means to place something down.
* "Herein" means "in this place" or "in this document," so it's used to emphasize the location or context of the statement.
Therefore, "herein lies" is the correct way to express that something is contained or present within a particular place or document.