We can infer from the passage that, in summer ______.

Two indelible impressions are left on a visitor to the Liseomb Bonebed polar dinosaur site, on the Colville River, northern Alaska: it is incredibly rich in fossil remains and it is an extremely dangerous place to excavate during the summer. The fossils occur in a band of permafrost - the ice-cemented rock characteristic of much of the Arctic. If one presses a finger at random onto this band, there is an almost fifty per cent chance that one will touch a dinosaur bone. The danger of working this site during the summer is the fact that in temperatures above 00C, the ice-cement that binds the permafrost together melts and it suddenly collapses without warning.