Defenders Vol 1 (1972) 15

Panic Beneath the Earth! Part 1 of 2. Guest-starring Professor Xavier. Script by Len Wein. Art by Sal Buscema (layouts) and Klaus Janson (finishes). Cover by John Buscema, Mike Esposito, and John Romita (touch ups). It's an epic encounter beneath the Carlsbad Caverns as Professor X, Dr. Strange, Nighthawk (in his new costume), Valkyrie, and the Hulk battle the Blob, Lorelei, Mastermind, Unus, and Magneto! Flashback cameos by Captain America, Thor and the Vision to recap Magneto's defeat in Avengers #111. (Notes: This story occurred during the 5-year period when the X-men series was in reprint. During this time Magneto battled Marvel's other superhero teams including the Fantastic Four, Inhumans, and Avengers. The Defenders title returns to monthly publication with this issue. The letters page contains Marvel Value Stamp series A #8 Captain America by John Buscema.) Cover price $0.25.