X-Men #64

Published in January 1970, X-Men #64 serves as a significant marker in the series' history as it represents one of the final issues before the title entered a prolonged period of reprint-only content. Written by Dennis O'Neil and illustrated by Don Heck, the issue follows the team's ongoing struggle against the Sub-Humanity, a subterranean civilization. This era of the series struggled with declining sales, leading Marvel to transition the book into a bimonthly reprint format starting with issue #67.

The cover art, provided by Neal Adams, is widely considered the highlight of this specific issue. Adams was instrumental in revitalizing the visual identity of the X-Men during this period, moving away from the more traditional house style toward a dynamic, cinematic aesthetic that emphasized character anatomy and dramatic lighting. While the issue itself reflects the creative uncertainty of the late 1960s X-Men run, it is frequently collected today as a bridge between the classic Silver Age stories and the dramatic transformation the franchise would undergo in the mid-1970s.