Foundation cracks like this are very serious and should be investigated thoroughly. They usually come in pairs so look for the other one. Uneven movement or settling of the foundation is the cause. Concerns for cracks of this nature are water or pest entry to the basement or crawlspace, secondary damage to finishes both interior and exterior, and ultimately structural failure is a possibility.

Surprisingly for a number of reasons in this particular hundred-year-old home it had little effect on the structure and only required monitoring to see if it was still moving.