This paper presents a case study of cliffside instability. Geotechnical analyses employing field measurements and laboratory test results obtained over a 30-yr period indicate that rising pore pressures trigger slides along the cliff. Visual observation, surveys, and field exploration show that most of the landslides have wedge-type failure geometry. Shear occurs through a nearly horizontal stratum of weak clay, brown fat clay. The strength of the fat clay decreases with deformation. Fluids generated by domestic and industrial activities have resulted in a perched water layer above the stratum of fat clay. For more than two decades, the pore pressure on top of the fat clay has risen. A wedge type of stability analysis can give the correct factor of safety — i.e., 1.0 — for the slides which have occurred.