Drive out of the underground parking lot, activate the Batmobile's Battle Mode and start pressing the Sonar key/button. Each time you use the Sonar look closely at the mini-map, as it will display roots leading to the Ivy's Plant. Follow the roots to the center - marked in the above pictures.
Stop the Batmobile and hold the Sonar button in order to send a special missile under the ground. This will launch another cut-scene.