The planet with the deepest and most massive atmosphere is Jupiter.

Here's why:

* Depth: Jupiter's atmosphere extends hundreds of kilometers deep, with no clear boundary between the atmosphere and the planet's core.

* Mass: The atmosphere is composed mostly of hydrogen and helium, and it accounts for a significant portion of Jupiter's total mass.

* Pressure: The pressure at the bottom of Jupiter's atmosphere is immense, reaching millions of times the pressure at sea level on Earth.

While other gas giants like Saturn, Uranus, and Neptune also have deep atmospheres, Jupiter's is the largest and most massive by far.