minecraft-assets/data/minecraft/worldgen/template_pool/colosseum/starts.json

{
  "elements": [
    {
      "element": {
        "element_type": "minecraft:single_pool_element",
        "location": "minecraft:colosseum/treasure/big_air_full",
        "processors": "minecraft:colosseum_veins",
        "projection": "rigid"
      },
      "weight": 1
    }
  ],
  "fallback": "minecraft:empty"
}