So close were the explosions that two U.N. military observers were forced to take refuge in their shelters at the southernmost outpost in the demilitarized zone on the strategic Golan Heights.

At first glance the sight of "UNMOs"--as the observers are called in U.N. jargon--heading for cover recalled October 1973. Then the Syrian Army was unleashing a massive armor assault in a nearly successful attempt to wrest back control of the Golan, lost to Israel on the last day of the 1967 Arab-Israeli war.

This time the explosions presaged no such renewal of hostilities. Instead, a brush fire raging unchecked had crept up to the perimeter of Observation Post 55, setting off long-forgotten land mines.

While it has been spared hostilities, however, the Golan Heights in recent weeks once again has been at the center of Middle East tensions.

In late May, Israel rushed more than a brigade of armor into the southern Bekaa Valley in eastern Lebanon in reaction to what it charged were major Syrian maneuvers near the Golan.

Specialists in Damascus say they are convinced the Syrians were conducting a very limited training exercise involving relatively few troops. But ever since 1973, when Israeli intelligence overlooked the massing of Syrian armor under the cover of maneuvers with nearly fatal consequences, the Jewish state has taken no chances when it comes to the Golan and its approaches.

Any major violation of the complicated disengagement agreement worked out here by former secretary of state Henry A. Kissinger would have major repercussions.

In the view of professional military men and senior diplomats, trouble on the Golan likely would signal a major war that could suck in both the United States and the Soviet Union in defense of their Israeli and Syrian allies.

Lately the Reagan administration has been dropping hints that it might be prepared to talk about the future of the Golan if the Syrians were to drop their outright rejection of the month-old Israeli-Lebanese agreement on withdrawing Israeli troops from Lebanon.

The Syrians have given no outward sign of agreeing to any such deal, which many observers here say would have to include an international conference with Soviet and Palestinian participation.

But reestablishing Syrian sovereignty over the entire Golan plateau remains a top priority for President Hafez Assad if only because he was Syrian defense minister when it was lost in 1967.

For years Syrians privately have toyed with the idea of declaring the whole plateau a demilitarized zone to calm Israeli fears of having the Upper Galilee region once again within Syrian artillery range.

Complicating any such solution was Israel's decision in December 1981 to annex the part of the Golan occupied by Israeli troops.

Israeli determination to stay put is underlined by two signs on the main road crossing just to the west of the once-prosperous Syrian city of Quneitra, dynamited by Israeli engineers just before they turned it over to Syrian jurisdiction in 1974.

One sign read "Stop--border check," the other "Welcome to Israel."

Dominating the ruins of now-deserted Quneitra--and the Syrian positions to the east--are Israeli-held hills, especially nearby Tal Abu Nada--Arabic for Hill of Father Dew--more commonly known to the U.N. force as Spy Hill because of the elaborate complex of radar and other electronic equipment clearly visible on its summit.

Similar Israeli electronic installations are clustered on Mount Hermon, the highest peak in the area, which provides a clear view for many miles for the Syrians and the Austrian U.N. troops.

Military specialists say this Israeli possession of so much high ground puts the Syrians at a tactical disadvantage in the event of any future hostilities.

Under terms of the 1974 agreement, the Golan is policed by no more than 1,290 men of the U.N. Disengagement Observer Force. So calm has the Golan remained militarily that not since November 1975 has the regular six-month renewal of the force by the United Nations seriously been questioned.

Driving around the Syrian side of the Golan, one gets the distinct impression that the Syrians are stationing far fewer troops than the 6,500 allowed within six miles of the cease-fire line under the arrangement.

Israel and Syria are allowed no troops within the so-called area of separation, which ranges from a width of 7.5 miles on the Mount Hermon ridgeline to 200 yards at its southernmost point 45 miles to the south.

On six miles either side of that zone each army is allowed 75 tanks and 36 artillery pieces, and in the next six-mile zone the limit is 450 tanks and 162 cannon. No surface-to-air missiles are permitted within 15.6 miles of the cease-fire lines.

To monitor compliance, aerial photographs are taken by U.S. Air Force planes based on Cyprus.

From a purely technical point of view, the U.N. force is hopelessly outclassed. It is made up of about 20 observers plus a battalion of Austrian troops in the hilly country north of the Quneitra-to-Damascus road and a Finnish battalion to the south and Canadian and Polish logistics and communications contingents.

Without radar, using only 1950s optics to augment its line-of-sight positions, the U.N. force is using many of the same techniques employed by Crusader forts here 700 years ago.

But the whole theory of deterrence is based on the hope that the exposed U.N. forces and observers--armed only with personal weapons--will have a crucial political impact on the world community in case of any fresh fighting.

During the windy summers and muddy winters, from the peaks of Mount Hermon, still dappled with snow, down to the narrow defile of the Wadi Raqqad River in the south, U.N. troops inspect the Israelis every other Tuesday and the Syrians every other Wednesday.

Each side, moreover, has the right to call a snap inspection within 24 hours. The most recent such move followed Israel's annexation decision.

But most of the time, in the words of one U.N. officer, the duty is "nice and boring." Calm is what the U.N. force thrives on, and judging by the number of Syrian farmers, cows, fig trees, wheat fields, donkeys and sheep--as well as the impressive amount of new government-financed construction--so do the local residents.

By the standards of most peace-keeping operations, the annual $34 million budget for the U.N. force is a bargain.

Looking over the plateau strewn with volcanic rock--some of it neatly piled in stone fences to allow farming--Austrian Maj. Horst Malat summed up his duties as "still chasing the same old shepherds and the same old sheep."

He should know. He was part of the first U.N. team in 1974 and now is on his third tour, this time as the Austrians' operations officer. As long as straying sheep and shepherds are responsible for the only major violations his men must deal with, he figures the U.N. observer force is serving its purpose.