In historic time the Phlegrean Fields near Naples have always been subject to elevation changes which have been recorded with various means of observation. The most remarkable change began in the year 1000 ad and terminated in 1538 with a large volcanic explosion and the formation of a hill. In 1970 geodesists noted that the elevation had increased again and the phenomenon was studied, observing several geophysical parameters. We report here the results of the repeated measurements of the exceptionally large horizontal strain made in the period 1970–77. Copyright © 1979, Wiley Blackwell. All rights reserved