Chick, Bryan. Secret zoo #6

"Hidden beneath the regular Clarksville City Zoo, the Secret Zoo is an incredible world where animals and humans peacefully coexist as equals. But even in the Secret Zoo, there are dangers and forbidden places. When Noah and his friends discover tarantulas roaming around, they're sure the bad omen can only mean one thing: the Shadowist is nearby. Closing the portals is the only option to save all the animals at Clarksville City Zoo...But when Mr. Darby and the Descenders are unsuccessful in destroying the portals, Noah realizes that the Secret Zoo is going to war"--