Marble Mounds are one of the gimmicks in Renaissance Age in the Chinese verison of Plants vs. Zombies 2. They contain statues, so you may not know what zombie could be in it. During the day, they act like high-health obstacles that absorb straight-shot projectiles. During the night, they will release the zombie they're containing. Sculptor Imp Zombie can chisel the Marble Mounds to release the zombie inside, but it takes a longer time because Marble Mounds are bigger than Statues.