format
A matrix with 3 columns and 510 rows. Each row corresponds to
the measurements of an earthquake beneath the Mt St Helens volcano.
The first column is the longitude (in degrees, where a negative number
indicates west of the International Date Line), the second column is
the latitude (in degrees, where a positive number indicates north of
the Equator) and the third column is the depth (in km, where a
negative number indicates below the Earth's surface).source
Scott, D.W. (1992). Multivariate Density Estimation: Theory, Practice
and Visualization. John Wiley & Sons Inc., New York.