Subsurface mineral fluids and the substances recovered from them constitute a major part of the value of all minerals produced in this country, increasing from about 48 per cent in 1946 to 58 per cent in 1961, not including ground water. Each mineral fluid has its own preferred habitats, and finding new sources will require ever-increasing knowledge of geologic principles and processes.

The predicted annual demand for petroleum and natural gas by the year 2000 is three to four times present domestic production. This increased demand must be balanced by increased rate of production from known fields, by new discoveries, by increased imports, or by synthetic products extractable from coal and oil-shale deposits, or by utilization of other energy sources.

Other natural gases that come from subsurface reservoirs include helium, carbon dioxide, and hydrogen sulfide. Helium is in particular demand because of its unique physical and chemical properties; its geologic habitat is becoming better known.

About one-sixth of the country's present water supply comes from ground water. In some areas withdrawal exceeds recharge, but in other areas withdrawal can be increased greatly without exceeding potential recharge. Currently about one-third of the ground water withdrawn is not being replaced. The behavior, quality, and quantity of both surface and ground water are geologically intimately interrelated. Increasing water usage will require improved scientific and legal coordination.

Subsurface saline waters pose a threat to some fresh-water supplies; but with improved conversion techniques saline water can provide additional fresh water. Some fossil brines are now rich sources of valuable chemicals, and other brines are potential sources.

Development of geothermal energy from subsurface thermal water and steam has begun, and further exploration will increase the power output. Recovery of valuable chemicals dissolved in some geothermal fluids is being considered.

New uses for low-value fluids include "fluidizing" solids for easier transport and handling, and "solution mining" of low-grade ores.