The project engineer had been confident that extensive use of grout—a mixture of cement and water—could seal the holes. However, the bureau's own grouting expert testified before Congress after the failure that grouting is not an exact science and that absolute certainty is impossible. Engineering is said to depend upon science, drawing on and applying scientific principles in its practice. Science, with its freedom to do pure research, seems to be at the top of a hierarchy among the institutions. The structure of science itself is hierarchical, headed by an elite corps, notably theoretical physicists. Science suggests a universe unfolding progressively over time, in causal sequence, under uniform laws that governed the past and will continue eternally. Science has claimed that the only reliable knowledge is obtained by the scientific method, which involves deduction, pulling hypotheses down from theories, and testing them in experiments under controlled conditions.