When you use `JSON.deserializeUntyped(data)`, you need to perform type casting to tell the Apex compiler what kind of object you're working with or you'll get errors because the type `Object` has no property `baseVolume` (e.g.).

One way to accomplish this would be:

    for (Object obj : meta.keyset()){
        Data d = (Data)meta.get(obj);
    
        System.debug(d.baseVolume);
    }