​Underground hole 6 feet x 6 feet x 6 feet encased in concrete blocks connecting to home.

Hole encased in concrete found underneath rock bed. Hole is approximately 6 feet x 6 feet x 6 feet and had a concrete cap as well. This hole leads directly into my unfinished basement, with a plastic sheet separating the basement and hole. The house is in Michigan and was made roughly in the 1950s. What could the intention of this hole have been and why cover it instead of filling it in?