"Ambush! Curse them!"
A wall collapses and all eyes turn to see an Antman caught in the torchlight of the dining hall, like a deer before a wolf. A long moment passes before someone grabs the nearest crossbow and fires a bolt into its torso with a sickening crack. Its screech resounds back through the tunnel and is answered by the vengeful cries of its brethren.
The guard is called and plate clanks hurriedly, charging forth to stop the invading tide. A violent battle ensues, leaving ichor and abdomen contents splattered across the tables as the guard pushes them back into the foul hole from which they ceaselessly stream, until they hear a sinister chitter.
A Giant Cave Spider, lured by the sound of battle and the scent of blood, leaps from the darkness and starts mauling your men; webbing catches their beards and covers their eyes, leaving them to scream blindly as they are torn asunder. The captain of the guard calls back down the tunnel and the frightened onlookers rush to the nearest stone stockpile, already dreading their appointed task. More men fall, their weapons stuck in the beast's horrible hide, unable to free themselves from the nightmarish entanglement, until only one remains; the captain, standing stalwart. He turns his head to see his teary-eyed companions before the final brick and the spider's fangs slide into place, sealing off the light forever.
Days later, an engraver stands up solemnly from his work and walks away, leaving a testament to the sacrifice of his fellows in the place where they laid down their lives. It is decorated with finely-crafted antman shell.
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And that is why digging enemies should be implemented as part of the spiced-up Underground Diversity bit.
See also: Tunneling monsters. MOOOOLES.