Easy.

  declare -A outer=(
    [inner]="_inner"
  )
  declare -A _inner=(
    [key]="value"
  )

Access inner elements via a nameref.

  declare -n inner="${outer[inner]}"
  echo "${inner[key]}"
  # value

Currently writing a compiler in Bash built largely on this premise.

Flatten the damn thing and process it relationally. Linear data scans and copying are so fast on modern hardware that it doesn't matter. It's counterintuitive for people to learn that flattened nested structure with massive duplication still processes faster than that deeply nested beast because you have to chase pointers all over the place. Unfortunately that's what people learn at java schools and they get stuck with that pointer chasing paradigm for the rest of their careers.

Sometimes a you just have to accept a language's limitations.

Try in Python to make a nested defaultdict you can access like the following.

    d = <something>
    d["a"]["b"]["c"]  # --> 42

Can't be done because it's impossible for user code to detect what the last __getitem__ call is and return the default.

Edit: Dang it, I mean arbitrary depth.