Stockholm — What has been dubbed here ''the submarine summer'' started when a periscope broke the turbid waters of the Gulf of Bothnia, just off Harnosand, where there is a coastal artillery unit.
The memory of last year's incident in which a Soviet Whiskey-class sub armed with nuclear warheads ran aground on rocks off Sweden's main southern naval base at Karlskrona was still fresh.
As worried civilians called in June 3, the Swedish Navy went into action. The submarine fled north. The Navy sealed off the gulf between Umea and Vaasa on Finland's west coast, leaving the sub only a narrow, shallow channel for escape. The incident, which coincided with a heatwave, kicked off Sweden's ''submarine summer.''
On June 4 a second submarine was spotted off the east coast industrial city of Sundsvall. A second ''search and identify'' mission swung into effect.
The next day a submarine was reported in the Stockholm archipelago. That submarine obligingly slipped back out into international waters.
But searches continued off Umea and Sundsvall, and a new sighting was made off Hallstavik June 8. Depth charges were dropped from a helicopter, and the intruder headed east rapidly, pursued by Viggen jets and naval patrol boats.
On June 11 a naval patrol boat monitored a sub off the Ahvenanmaa Islands, and a depth charge was dropped.
Three days later there was a rerun of this incident a little farther north. This hunt continued for an hour, and four depth charges were dropped.
During this frenzy of activity up to 200 men were on alert, and the Swedish Navy's eight helicopters were touching down only to refuel before going back out on antisub patrol.
By June 16 the Navy had to scale down its operations. The area off Umea returned to ''a normal state of preparedness.'' Antisub missions continued to the south.
A Defense Ministry spokesman said it was uncertain how many submarines had been sighted. Several sightings may have been made of a single submarine, Bertil Lagerwall, the ministry's press spokesman, said. Only the June 8 sighting had been positively identified as a submarine, he said.
Although the ''submarine summer'' seemed to be petering out along with the sunshine, it has posed grave questions about Sweden's coastal defense systems. Mr. Lagerwall admitted: ''Our resources are severely limited.''
And Defense Minister Torsten Gustafsson promised the Navy would be given new weapons to deal with further incidents of this kind. He described the present situation as ''extremely unsatisfactory.''
One new weapon the Navy will be getting is a magnetic charge that can be dropped from a helicopter to fasten to a submarine and blast a small hole in its superstructure, flooding the section immediately underneath and, it is hoped, forcing the vessel to surface and identify itself.
Officially the Defense Ministry describes all the submarines as ''unknown.'' But independent defense experts here say there is little doubt they are Soviet. They say the recent incidents have resulted from a Soviet spying operation aimed at charting Swedish east coast military installations.
''It is probably a follow-up to last year's south coast spying mission that led to the Whiskey incident,'' one expert said.
Before ''the submarine summer,'' there had been six confirmed sightings of unknown submarines in Swedish waters since Whiskey ran aground.
As for the future, once the sun returns no fishing trip or boat ride will be complete without passengers eagerly scanning the horizon for periscopes.
The Falklands war and the Israeli blitzkrieg on Lebanon may be front-page news here, but there is nothing like having something happening in your own backyard to keep you on your toes.
There is concern that this all might turn into more than a joke to brighten a dull summer. Just what happens if Sweden depth charges a nuclear-armed Soviet submarine is unclear.
And as for the possibility of capturing another Soviet sub, nobody was prepared to comment on what sort of diplomatic action would be taken, much less what the Soviet reaction might be.
But at the height of ''submarine summer,'' optimistic officials in Umea discussed plans to set up an international press center. Local traders, remembering the unexpected tourist boom in Karlskrona last fall, began stocking up.
Mysterious periscopes may be bad for the politicians, but they are certainly good for business in this neck of the pine forests.