Full Metal Panic! Volume 10

The battle at Niquelo ignited in Sousuke and Kaname a new passion to find each other, but the rest of the world won't make it that easy. While Sousuke fights, waiting for the operation that might bring her back to his arms, Kaname endures new trials at the hands of a jealous tormenter. Everything comes to a head in an abandoned Soviet research town, and the winding tunnels beneath it—tunnels that hosted a catastrophe seventeen years ago, and which might hold the secret to the Whispered themselves!

[From J-Novel Club]