What do the boys accidentally do in Chapter Two of Lord of the Flies by William Golding?

The boys accidentally set fire to part of the island as they build a rescue fire.

After Ralph waves the conch, the boys are calmed by his assurance that they will be rescued because, as the son of a Navy commander, he knows that sooner or later a ship will come near enough to the island. But, he adds, the best thing to do is to build a rescue fire, as the ships will easily...

The boys accidentally set fire to part of the island as they build a rescue fire.

After Ralph waves the conch, the boys are calmed by his assurance that they will be rescued because, as the son of a Navy commander, he knows that sooner or later a ship will come near enough to the island. But, he adds, the best thing to do is to build a rescue fire, as the ships will easily see the smoke and know that the island is inhabited.

The boys enthusiastically shout, "A fire! Make a fire!" They go down about a hundred feet below them where there is some dead wood and gather it. Enthusiastically, the boys drag dead wood and pile it excitedly on the point of the mountain where they have been.

Ralph and Jack looked at each other while society paused about them. The shameful knowledge grew in them and they did not know how to begin confession. 

Ralph asks Jack to light the fire. They are not sure how to do this, so Roger calls out that they must make a bow and spin the arrow. Just then, Piggy comes up the side and Jack shouts, "His specs--use them as burning glasses!" The boys surround Piggy and take his glasses. Jack blows on the fire after Ralph waves the glasses back and forth igniting the old, rotten wood. The boys cheer and dance in glee. However, because the pile is so rotted, the fire blazes out of control as 

[L]ife became a race with the fire and the boys scattered through the upper forest. To keep a clean flag of flame flying on the mountain was the immediate end and no one looked further....Acres of black and yellow smoke rolled steadily toward the sea.

The reality of destruction becomes clear to the boys. After the fire goes out, no one can find the boy with the mulberry scar, so the fire becomes symbolic of the power of brute force.