History of the Peloponnesian War

The Peloponnesians, on perceiving this, rammed down clay in wattles of reed, and threw it into the breach, that it might not be loose, and so carried away like the soil.

Being thus baffled, the Plataeans ceased from this attempt; but having dug a passage under ground from the city, and having guessed their way under the mound, they began again to carry the soil in to them. And for a long time they escaped the observation of the enemy outside; so that though they continued to throw on materials, they were further from finishing it; as their mound was carried away from beneath, and continually sinking down into the vacuum.