The mining of San Juan Puerto Rico as reported in Wikipedia appeared to be an unsubstantiated rumor until the author obtained the KTB, or attack report, of U-218 under Richard Becker (Uboat.net had already reported it). This submarine was a mine-laying war horse which laid its delicate and deadly eggs off Trinidad, St Lucia, Puerto Rico, England and Scotland during its career. On the patrol in question U-218 entered the region by rounding St Martin and Anegada on the 30th of March, 1944. From there it motored due west to just off San Juan.
The exact timing and quantity of its mining operation has been variously reported, with Wynn saying 12 mines were laid on April 1st and Uboat.net claiming that 17 mines were laid on the 1st of April and five more on the 3rd. According to the actual attack report, Becker logged that on April 1st he laid two MTA-type mines off San Juan and fifteen SMA type. Furthermore on the 3rd of April (German time) he reported that U-218 laid five EMS mines, which stands for standard drifting mine (Einheitsmine Sehrohr–Treidmine). This would appear the conclusive primary source evidence that twenty-two mines were laid over a three-day period.
In any event, none of the mines struck any Allied vessel and the record is silent as to whether any of the mines were detected at all. In all probability they eventually broke free and drifted – or are still drifting – west with the prevailing currents and winds towards the Bahamas and then circled around the Sargasso Sea.
U-218 sailed northeast of San Juan on the evening of the 3rd of April, then on the 4th turned east and exited the area on the 5th for a total patrol in the region of one week. It returned to Brest on the 7thof May, having set out from that port on the 12th of February. The submarine was “driven off by aircraft” according to Wynn from both St Lucia and San Juan. In fact it is impressive that a submarine managed to get close enough to either Castries or San Juan in mid-1944 at all, given the strengthened anti-submarine forces in place by that time.
Kapitänleutnant Becker was a member of the Crew of 1934. His total tonnage sunk was one Allied ship for 146 tons, an auxiliary warship of 7,177 tons, and a ship damaged for 7,361 tons. He undertook an impressive eight patrols for 372 sea days. Perhaps more impressively, despite having then taken over U-2503 in October 1944, Becker managed to survive the war. Thirty-three years of age at the time of this patrol, Richard Becker is still alive and over 100 years of age at this writing. His highest decoration was the U-boat Front Clasp to the German Cross in Gold, awarded following this patrol.
SOURCES: Gudmundur Helgason, Rainer Kolbicz, www.uboat.net, 2011, Kenneth Wynn, U-boat Operations of the Second World War, Volume 1 and Volume 2, 1997