JED 1.6.0+ - Bedrock-capped full stone, early register

{
    "dimensions": [
        {
            "dim": 8,
            "normal_biomes": true,
            "load_on_start": true,
            "dimensiontype": {
                "id": 8,
                "name": "Underworld",
                "suffix": "_dim8",
                "keeploaded": false,
                "worldprovider": "WorldProviderSurfaceJED",
                "require_exact_match": true
            },
            "worldinfo_onetime": {
                "generatorName": "flat",
                "generatorOptions": "3;1*minecraft:bedrock,254*minecraft:stone,1*minecraft:bedrock;1;"
            }
        }
    ]
}