Adventure Comics, Vol. 1 #409 (1971)

Invasion Of The Mer-Men! / Fight With Fire Drake / The Condemned Legionnaires! / The Secret Of Satan Girl!
"Invasion of the Mer-Men (Part II)": Supergirl looks underwater for her co-worker Johnny and her captors, the whole time blaming herself for her co-worker's demise. Finally she finds the secret base of the Mer-Men, and discovers they are loading giant "tanker" space-globes with water to be shipped to their home planet. In the meantime, a coalition of Earth armies launches attack after attack to destroy the globes, but they have force-fields that resist any attempt Earth makes to destroy them. Supergirl locates Johnny. A patrol of Mer-Men attacks her and she lets herself to be captured.  (This story is reprinted from Adventure Comics #313.)
Release Date: 1971