Water-tube tiltmeter installed in 1956 in an underground vault near...

The Uwekahuna WT (water-tube) instrument is still measured today, providing one of the longest records of deformation for any volcano. The tiltmeter is read by looking into the portal on each pot and adjusting the sight until two pins appear to touch. In fact, one of the pins is a reflection, so having the pins almost touch establishes the exact water level within the pot. The difference in water level gives the tilt, and repeated measurements give changes in tilt over time.