Your instrument could not have registered properly; perhaps it never went all the way down; and perhaps it is all a mistake.

It may be that the lead did not go down so far as you think." He smiled; he was becoming calmer now, for he was doing something: he was obtaining results.
"Those ideas about increasing heat at increasing depths are old-fashioned, Margaret," he said.

"Recent science has given us better theories.

It is known that there is great heat in the interior of the earth, and it is also known that the transmission of this heat towards the surface depends upon the conductivity of the rocks in particular locations.

In some places the heat comes very near the surface, and in others it is very, very far down.