Team members are testing the hypothesis that at one time a much thicker atmosphere surrounded planet earth and that the destruction of this "canopy" accounts for many geological features that we observe today. Opinions differ as to the elements in the canopy. Was it water vapor? Ice? Was the atmospheric pressure higher long ago? Was there a higher oxygen content? (Science, Vol. 239, p1406) Research continues on an on-going basis to answer these and other questions.