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Midway between Nes Ziona and Rehovot on a tree-covered hill is one of Israel's best-kept secrets.

Known as the Ayalon Institute, it was only in 1975 that its existence became public knowledge, even though its operation ceased in 1948.

The story of the Ayalon Institute begins in 1945 and covers the years between the end of World War II and the establishment of the state of Israel.

In 1945 Palestine was ruled by the British, whose policy was to ensure that a very strict quota of Jews — survivors of the Nazi Holocaust — was allowed to come to Palestine and that the fledgling Jewish underground, the Haganah, was outlawed and starved of weapons and ammunition.

The question of ammunition was a crucial one, as the standard personal weapon of the Jewish underground at the time was a domestically produced Sten submachine gun. The 9 mm bullets needed for the gun were very hard to come by, and the British Mandate security forces did everything they could to ensure that the Jews did not get their hands on weapons or ammunition.

Yosef Avidar was, in 1945, the head of the clandestine Israel Military Industry. The function of the IMI was to manufacture or procure from abroad weapons and ammunition for the Haganah.

The pre-state leaders knew that once Israel became independent, it would be necessary for the country's survival that the Haganah had sufficient reserve of guns and ammunition. It was as a result of this need that one of the most imaginative and secret military projects ever undertaken took shape right under the watchful eyes of the British.

Avidar, together with the head of the Haganah, Eliahu Golomb, devised a plan to build an ammunition factory about 13 feet under the ground to secretlymanufacture bullets for the Sten gun. The digging and casting work of the nearly 2-foot-thick walls and ceiling was completed in a mere 22 days

Under the code name "the Ayalon Institute" a group of pioneers from the Hatzofim Aleph movement and people from IMI built and began operating the facility only a short distance from a nearby British military camp.

It was at the time the largest clandestine weapon-making factory in Palestine. Between 1945 and 1948 it produced more than 2 million 9 mm bullets.

In order to disguise the fact that the IMI was manufacturing ammunition in the factory, the project was run above ground as a fully operational kibbutz in the area known as Kibbutz Hill. So secret was the factory that newer members of the kibbutz were unaware of the manufacturing operation going on beneath their feet.

The project had all the trappings of an ordinary kibbutz with living quarters, a kindergarten, a chicken coop, a cow barn and a vegetable garden. Above the factory was a laundry and a bakery that, besides serving the kibbutz and the immediate area including the British camp and Rehovot, camouflaged the ammunition factory below ground.

The underground plant, which was about the size of a tennis court, needed two important things to function: air to breathe and cover for the noise produced by the machinery.

The flow of clean air was introduced to the factory through pipes that were attached to the bakery furnace. The polluted air was discharged through the laundry. The bakery in particular needed big air pipes to feed the fans in the ovens, so nobody thought twice about the inlet pipes on the bakery roof.

The churning of the big drums of the laundry and the clatter of the bread-making machinery covered the noise underground. At the same time the two small buildings housing them were also functioning as the entry and exit point for the factory below. The oven of the bakery, which weighed many tons, slid aside on metal runners to allow the workers access. The machinery holding the main drum of the commercial washer was also swung on a pivot to allow access underground.

Forty-five people worked below ground in two shifts. Work was hard, and the long hours underground were the cause of a situation that could have given away the existence of the factory. Because of the long, hot, dry, sunny summers in Israel and the mild winters, people who work in kibbutzim usually look tanned and healthy just from being in the open air. The workers laboring below ground day after day looked tired and pasty. The problem was solved by installing a sun lamp in a room below ground so that the workers could take turns getting a kibbutz tan and thus blend in with the agricultural workers.

The machinery need to produce the bullets was purchased in Poland and smuggled to Beirut and then onward to the factory. As the ammunition was manufactured it was quickly hauled up above ground at night and transported in the hidden compartments of milk trucks to arms caches around the country. Such was the excellence of the organization that the British never knew what was going on beneath their noses.

In 1948, following the foundation-laying of the state of Israel, the underground factory was closed and the production was transferred to other factories all over Israel. The pioneer group from the Ayalon Institute decided to stay together and established a new kibbutz, Kibbutz Ma'agan Micha'el, by the sea near Zichron Ya'acov.

In 1987 the factory and the immediate facilities connected with it were restored and turned into a museum. Today visitors can descend with ease to the factory by means of a newly installed spiral staircase alongside the now cold ovens and see how this remarkable operation was carried out. This best-kept secret was probably one of the single most important operations that prevented the Arab armies from overrunning the fledgling state at its inception.