The first tunnel was completed in 1867. With the growth of the city other tunnels and cribs have been built, farther out in the lake, to supply the increasing need. By 1870 Chicago had become one of the largest cities in the country. In 1830 the settlement at the mouth of the Chicago River had barely twenty houses. Forty years later it had over three hundred thousand inhabitants. The wonderful resources of the upper Mississippi valley had been largely responsible for the city's growth, and the rapid development of the entire West promised Chicago a still greater future. Then came the fire, and to the homeless people looking across miles of blackened ruins it seemed that Chicago had no future at all. Had not the fire undone the work of forty years? [Illustration: CLARK STREET IN 1857] The first despair gradually gave way to a more hopeful feeling. Truly the loss was great--the best part of the city lay in ruins. But was not the wealth of the West left, and the harbor and the railroads? These had built up Chicago in the beginning, and they would do so again. The rebuilding began at once. At first little wooden houses and sheds were constructed to give temporary shelter to the homeless. Help came to the stricken city from all sides. Thousands of carloads of food were sent, and several million dollars were collected in Europe and America. Two thirds of the city had been built of wood. Now the business blocks, at least, were to be as nearly fireproof as possible. Tall buildings of brick and stone were planned. But such structures are heavy, and if they were built directly on the swampy ground underlying the city, there would be danger of their settling unevenly and possibly toppling over. So layers of steel rails crossing each other were sunk in the ground, and the spaces between them were filled in with concrete. Upon this solid foundation the first skyscrapers of Chicago were built. To-day concrete caissons are constructed on bed rock, often from 100 to 110 feet below the surface, and upon these rest the steel bases of the modern Chicago skyscrapers. Work went on quickly. In a year the business section was rebuilt. In three years there was hardly a trace of the fire to be seen in the city, which was larger and more beautiful than before.