The minefield that Seaview encountered is reused footage from the original Voyage to the Bottom of the Sea (1961). The people on the Seaview didn't see the mines until they were right in front of them. Then even though Captain Crane called "all stop," one of the mines was hung up on the vessel. The mini-sub was then sent out to cut the mine loose, but it struck another of the mines and exploded, destroying the mini-sub and damaging the ship. The rest of "Submarine Sunk Here" was different, including the reason for not noticing the mines before it was too late.
This episode takes place in the 1970s.