There's treasure in these lands - sometimes piles of City Cash and Bloonstones, but if you're really lucky you may find a special item. Look for land that has a gold chest on it, and capture that land to get the chest.
A Treasure Chest is an item placed on some tiles in Bloons Monkey City. These tiles can be in any terrain and will be covered with a chest. Upon completion of these special tiles, the player will get a normal reward for completing the tile plus a boosted reward from the treasure chest. It can contain a large variety of rewards such as Monkey Knowledge Packs, City Cash, Bloonstones, and Special Items that can boost the player's city no matter what level it is.