Type K copper tubing is the ________ and typically used for underground water service.

Type K copper tubing is known for its thick walls, making it the thickest type of copper tubing available. This characteristic provides it with superior durability and strength, which are essential for applications like underground water service where the piping may be subjected to soil pressure, potential corrosive conditions, and other environmental factors. The additional wall thickness helps prevent damage and ensures a longer service life in these demanding conditions.

In plumbing applications, the thickness of the tubing is crucial; thicker walls mean higher resistance to physical impact and external stresses, making Type K suitable for situations where longevity and reliability are critical.