From Super Detective Picture Library #179, August 1, 1960.
The year is 1942. High-flying recon planes spot a very unusual site of several German ships in a big circle with diving operations underway off Crete. They are looking for a wrecked bomber which had been carrying a shipment of sophisticated submarine-detection devices known as "Selectron". A Greek agent for the Allies reports the real location for the downed plane is on land and John Steel is sent to find it. Whether the Greek can be trusted is another matter.