A carpenter’s level can be like a swimming pool.
In order to build a carpenter’s level, a small clear tube is filled with colored liquid and an air bubble is left in the tube. Two visible lines, or marks, are placed on the small tube. The tube is then incorporated into a larger block of wood or metal. When this block is level, or very close to level, the air bubble will stay between the two lines. When the block is moved off of level, the air bubble will move past one of the lines. When the bubble moves past one of the lines, the device isn’t level. The device works on the principle of gravity.
This same principle can be used in inspecting a swimming pool. Pool builders presumably use a leveling device to ensure that the pool tile at the top of the water is all placed at the same elevation. If this is done, then the same amount of tile will show above the water all around the pool. If after a period of time some of the tile at the water level starts to show more or less tile, then this can be an indication that the soil, and the pool, may be either settling or moving upwards. Such soil movement can be an indication the further investigation is warranted into the soils or the building pad on the lot.