The beast had landed next to one of the towers, damaging it. Not critically, but still posing a threat to it. They had immediately dispatched a pair of Leviathans to deal with the colossal beast, along with several underground units when it became clear that whatever had arrived was tunneling, and tunneling fast.
The Burrowers reached it first, the first team realising they'd struck the thing when they were jolted to a halt, and were greeted by a roar of indignation. Coordinating when and where they struck the beast, the teams managed to turn it around, forcing it to breach the surface once again. When it emerged, the leviathans were waiting.
The battle was swift and unbalanced. The first leviathan fell to a barrage of spike-like missiles fired from the beast, the second to the earth shaking beneath its feet, knocking it off balance, before the beast impaled it with the pointed ends of its legs. The Burrowers disgorged their troops, but to no avail, the beast just ignored their weapons fire, or trod on them as it returned to its tunnel. The Burrowers followed it, but one by one they were singled off and destroyed. The silence of their response team reached the Chimera command center in the area, the lack of communication coming through telling them of the danger.
They found stragglers from the brief conflict, and tried to reach the beast, to chase it down its burrow, but it went so deep into the Earth, down into areas of extreme heat, and Chimera were never known for their heat resistance. In the end, it was dismissed. Surely, nothing could survive that heat. And even if it was at home in the core, alive, why would it ever bother the surface dwellers? The incident was recorded, but never thought much of. It was only millennia later that it mattered to anyone, or anything, on the surface.