That is a honeycomb. What you are looking at is the course aggregate. The concrete was not vibrated properly so the voids were not filled by the "soupy" part of the mix (the cement, fine aggregate - sand - and water).

Make sure you check the inside of the foundation wall at this location. Most honeycombs are only near one surface but I had one a few months ago that extended all the way through the foundation wall and leaked. The inside of the wall was not visible due to the sellers hiding it behind an insulated wall.

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Bruce Breedlove
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