script.on_event(defines.events.on_player_built_tile, function(event)
    -- there seems to be an inconsistency in the surface_index, https://forums.factorio.com/viewtopic.php?t=46502
    if event.surface_index + 1 == game.surfaces["nauvis"].index then
        local tiles = {}
        for i = 1, #event.positions do
            if game.surfaces["nauvis"].get_tile(event.positions[i].x, event.positions[i].y).name == "cave-ground" then
                table.insert(tiles, {name="grass", position = event.positions[i] })
            end
        end
        game.surfaces["nauvis"].set_tiles(tiles, true)
    end
end)