Step right up. In this bottle is a remedy of wonderful efficacy. Its curative powers are calculated to remove pain and alleviate human suffering and disease. Distilled 400 feet below the earth's surface, this remarkable liquid is Mother Nature's bounteous gift of healing. It is my pleasure - no, it is my duty - to bring this soothing restorative, this blessed ointment, this modern-day balm of Gilead to the public.

There were few takers of the 19th century elixir that came to be called "snake oil." It was one of the less successful uses of petroleum, but not the first to claim healing properties. Ancient Persians, 10th century Sumatrans and pre-Columbian Indians all believed that crude oil had medicinal benefits. Marco Polo found it used in the Caspian Sea region to treat camels for mange, and the first oil exported from Venezuela (in 1539) was intended as a gout treatment for the Holy Roman Emperor Charles V.

The mysterious oil that sometimes seeped to the earth's surface had other uses as well. In Mesopotamia around 4000 B.C., bitumen - a tarry crude - was used as caulking for ships, a setting for jewels and mosaics, and an adhesive to secure weapon handles. Egyptians used it for embalming, and the walls of Babylon and the famed pyramids were held together with it. The Roman orator Cicero carried a crude-oil lamp. And, in North America, the Senecas and Iroquois used crude oil for body paint and for ceremonial fires.

Crude oil - as petroleum directly out of the ground is called - is a remarkably varied substance, both in its use and composition. It can be a straw-colored liquid or tar-black solid. Red, green and brown hues are not uncommon. The image of James Dean dripping with black oil from his Texas gusher in the 1956 movie "Giant" may have been compelling, but it's not descriptive of today's oil producers. For one thing, the days when a gusher signaled a big discovery are long gone. Since the 1930s, oil producers have used blowout preventers to stop gushers. In addition, not all crude oils behave in the Hollywood manner. Some flow about as well as cold peanut butter.

Until the late 19th century, an oil find often was met with disinterest or dismay. Pioneers who settled the American West dug wells to find water or brine, a source of salt; they were disappointed when they struck oil.

Several historical factors changed that. The kerosene lamp, invented in 1854, ultimately created the first large-scale demand for petroleum. (Kerosene first was made from coal, but by the late 1880s most was derived from crude oil.) In 1859, at Titusville, Penn., Col. Edwin Drake drilled the first successful well through rock and produced crude oil. What some called "Drake's Folly" was the birth of the modern petroleum industry. He sold his "black gold" for $20 a barrel.

Petroleum was prized mostly for its yield of kerosene until the turn of the century. Gasoline was burned off, and bitumen and asphalt (the heavier parts of crude oil) were discarded. But gradually rising in importance were the incandescent light and the internal combustion engine. The former relied on oil-fired generating plants; the latter, on gasoline.

By the 1920s, crude oil as an energy source - not just as a curiosity - came into its own. But to many, it's still as mysterious as it was to ancient man. Even in the petroleum industry, most people never see crude oil.

Geologists generally agree that crude oil was formed over millions of years from the remains of tiny aquatic plants and animals that lived in ancient seas. There may be bits of brontosaurus thrown in for good measure, but petroleum owes its existence largely to one-celled marine organisms. As these organisms died, they sank to the sea bed. Usually buried with sand and mud, they formed an organic-rich layer that eventually turned to sedimentary rock. The process repeated itself, one layer covering another.

Then, over millions of years, the seas withdrew. In lakes and inland seas, a similar process took place with deposits formed of non-marine vegetation.

In some cases, the deposits that formed sedimentary rock didn't contain enough oxygen to completely decompose the organic material. Bacteria broke down the trapped and preserved residue, molecule by molecule, into substances rich in hydrogen and carbon. Increased pressure and heat from the weight of the layers above then caused a partial distillation of the organic remnants, transforming them, ever so slowly, into crude oil and natural gas.

Although various types of hydrocarbons - molecules made of hydrogen and carbon atoms - form the basis of all petroleum, they differ in their configurations. The carbon atoms may be linked in a ring or a chain, each with a full or partial complement of hydrogen atoms. Some hydrocarbons combine easily with other materials, and some resist such bonding.

The number of carbon atoms determines the oil's relative "weight" or density. Gases generally have one to four carbon atoms, while heavy oils and waxes may have 50, and asphalts, hundreds.

Hydrocarbons also differ in their boiling temperatures - a key fact for refiners who separate the different components of crude oil by weight and boiling point. Gases, the lightest hydrocarbons, boil below atmospheric temperature. Crude oil components used to make gasoline boil in the range of 55 to 400 degrees Fahrenheit. Those used for jet fuel boil in the range of 300 to 550 degrees, and those for diesel, at about 700 degrees.

There are three essentials in the creation of a crude oil field:

• First, a "source rock" whose geologic history allowed the formation of crude oil. This usually is a fine-grained shale rich in organic matter.
• Second, migration of the oil from the source rock to a "reservoir rock," usually a sandstone or limestone that's thick and porous enough to hold a sizable accumulation of oil. A reservoir rock that's only a few feet thick may be commercially producible if it's at a relatively shallow depth and near other fields. However, to warrant the cost of producing in more challenging regions (the Arctic North Slope, for example) the reservoir may have to be several hundred feet thick.
• Third, entrapment. The earth is constantly creating irregular geologic structures through both sudden and gradual movements - earthquakes, volcanic eruptions and erosion caused by wind and water. Uplifted rock, for example, can result in domelike structures or arched folds called anticlines. These often serve as receptacles for hydrocarbons. The probability of discovering oil is greatest when such structures are formed near a source rock. In addition, an overlying, impermeable rock must be present to seal the migrating oil in the structure.

The oldest oil-bearing rocks date back more than 600 million years; the youngest, about 1 million. However, most oil fields have been found in rocks between 10 million and 270 million years old.

Subsurface temperature, which increases with depth, is a critical factor in the creation of oil. Petroleum hydrocarbons rarely are formed at temperatures less than 150 degrees Fahrenheit and generally are carbonized and destroyed at temperatures greater than 500 degrees. Most hydrocarbons are found at "moderate" temperatures ranging from 225 to 350 degrees.

It is the particular crude oil's geologic history that is most important in determining its characteristics. Some crudes from Louisiana and Nigeria are similar because both were formed in similar marine deposits. In parts of the Far East, crude oil generally is waxy, black or brown, and low in sulfur. It is similar to crudes found in central Africa because both were formed from nonmarine sources. In the Middle East, crude oil is black but less waxy and higher in sulfur. Crude oil from Western Australia can be a light, honey-colored liquid, while that from the North Sea typically is a waxy, greenish-black liquid. Many kinds of crudes are found in the United States because there is great variety in the geologic history of its different regions.

Crude oil is a surprisingly abundant commodity. The world has produced some 650 billion barrels of oil, but another trillion barrels of proved reserves have yet to be produced. An additional 10 trillion barrels of oil resources await development, assuming the price of oil someday justifies production. These resources include bitumen, shale oil and oil in existing fields that might be produced through enhanced recovery methods.

Talk of crude oil oozes with superlatives. Not only was crude oil the basis of the world's first trillion-dollar industry, it also is the largest item in the balance of payments and exchanges between nations. And it employs most of the world's commercial shipping tonnage.

Crude oil may not be the panacea that snake oil claimed to be. But for 20th century industrialized nations, it has proved to be more than good medicine.