Sinkholes, Caves, Caverns

Isabel has plans to plant a vegetable garden in the backyard.  That is fine with Maxcine; she even pitches in to help turn the soil.  What the sisters turn up is a stone statue, but it sinks into the ground before they can grab it.  Not to be defeated, they start digging in earnest… right up until the time when a sinkhole forms and Isabel nearly falls into the bowels of the Earth.  It doesn't take many days to discover the entire neighborhood may be heading for disaster.