If you plumb something mysterious or difficult to understand, you succeed in understanding it.

If something is plumb in a particular place, it is exactly in that place.

When someone plumbs a building, they put in all the pipes for carrying water.

Plumb is used to give emphasis to an adjective or verb that describes a feeling or condition.

If someone plumbs the depths of an unpleasant emotion or quality, they experience it or show it to an extreme degree.

If you say that something plumbs new depths, you mean that it is worse than all the things of its kind that have existed before, even though some of them have been very bad.