What does "deep down below " mean?
All the most important rooms (in my chocolate factory) are deep down below the surface!

Question

Answer

below the surface = underneath, or underground, but how far down is not specified.
down below the surface = some distance under the surface
deep down below the surface = a long way under the surface

They are approximately describing an order of magnitude, where 'deep down' is similar to 'very deep'.

Don’t miss out on the opportunity to learn a language from the comfort of your own home. Browse our selection of experienced language tutors and enroll in your first lesson now!