The forces behind the relentless land movement plaguing Rancho Palos Verdes appear to be more complicated, and possibly more widespread, than originally thought — forcing the city to once again rethink how it can limit some of the mounting damage. Officials confirmed a very deep and active landslide plane — previously considered dormant — during exploratory drilling this summer, a discovery that has upended emergency efforts aimed at stabilizing the Portuguese Bend area. … City officials last week voted to move ahead with a pilot plan to drill vertical wells into the deeper landslide, with the aim of pumping out groundwater — known to be the impetus for the land movement — while continuing to study the extent and characteristics of the deeper slip plane.