subterrane.defaults = {
    --y_max = Must be provided by the user
    --y_min = Must be provided by the user
    cave_threshold = 0.5,
    warren_region_threshold = 0.25,
    warren_region_variability_threshold = 0.25,
    warren_threshold = 0.25,
    boundary_blend_range = 128,
    perlin_cave = {
        offset = 0,
        scale = 1,
        spread = {x=256, y=256, z=256},
        seed = -400000000089,
        octaves = 3,
        persist = 0.67
    },
    perlin_wave = {
        offset = 0,
        scale = 1,
        spread = {x=512, y=256, z=512}, -- squashed 2:1
        seed = 59033,
        octaves = 6,
        persist = 0.63
    },
    perlin_warren_area = {
        offset = 0,
        scale = 1,
        spread = {x=1024, y=128, z=1024},
        seed = -12554445,
        octaves = 2,
        persist = 0.67
    },
    perlin_warrens = {
        offset = 0,
        scale = 1,
        spread = {x=32, y=12, z=32},
        seed = 600089,
        octaves = 3,
        persist = 0.67
    },
    --solidify_lava =
    columns = {
        maximum_radius = 10,
        minimum_radius = 4,
        node = "default:stone",
        weight = 0.25,
        maximum_count = 50,
        minimum_count = 0,
    },
    --double_frequency = -- when set to true, uses the absolute value of the cavern field to determine where to place caverns instead. This effectively doubles the number of large non-connected caverns.
    --decorate = -- optional, a function that is given a table of indices and a variety of other mapgen information so that it can place custom decorations on floors and ceilings.
}

local recurse_defaults
recurse_defaults = function(target_table, default_table)
    for k, v in pairs(default_table) do
        if target_table[k] == nil then
            target_table[k] = v -- TODO: deep copy if v is a table.
        elseif type(target_table[k]) == "table" then
            recurse_defaults(target_table[k], v)
        end
    end
end

subterrane.set_defaults = function(cave_layer_def)
    recurse_defaults(cave_layer_def, subterrane.defaults)
end