Caissons (or piers) are a type of a deep foundation, whereby a hole is excavated in the ground, and a reinforced steel structure is built above ground level, and is then sunk into the hole.  Cement concrete is then poured into the caisson to add strength, durability, and weight.  Caissons can be a good way to secure a foundation to bedrock for hillside homes or homes built on an “active” soil such that they can sink or slide (e.g., during an earthquake, mudslide, or landslide).