The underwater excavations quickly encouraged the archaeologists to pose questions as to the conditions that formed the sunken sites. Comparisons with the ruins unearthed in the land digs led them to suppose that Alexandria had been the victim of subsidence, that the Earth's crust had sunk to some great depth: five to six metres. To study this specific phenomenon, a team of geographers and marine biologists began in 1997 to examine micro-fauna both underwater and in the silt deposits on either side of the Heptastadion. A series of cores have been taken since 1998 in order to study the variations in sea level and changes in the coastal ecosystem.