Deck footer
If you’re not familiar with the construction process, then you might be just as flabbergasted as the OP was when they came across this concrete block. He initially thought that it was a part of his home’s septic tank, but without pipes, his suspicions were proven wrong. So, what is it, actually?

This concrete block goes by the name of deck footer. It sits below one of the four (or more) legs of a deck to keep the wooden construction stable over time. If you were to plant the legs directly into soil, your deck would sink deeper and deeper into the ground over time.