Shimla, Sep 23 (IANS): Prime Minister Narendra Modi will inaugurate the strategic Atal Tunnel, earlier known as the Rohtang tunnel, on October 3 in Kullu district, ahead of which Himachal Pradesh Chief Minister Jai Ram Thakur reviewed the preparations on Wednesday.

Once in use, it would be the world's longest tunnel at an altitude of 10,000 feet above sea level, costing Rs 3,500 crore.

The tunnel -- extremely significant from the military logistics point of view -- was also expected to boost the socio-economic development of the people of Lahaul-Spiti.

The Chief Minister said that it would provide new opportunities in the agriculture, tourism and horticulture sectors, which would change the economy of the district.

While the tunnel was originally designed to be 8.8-km long, fresh GPS readings taken by the Border Roads Organisation (BRO) showed that it is 9 km in length.

The tunnel provides telephone facility at every 150 metres, fire hydrant at every 60 metres, emergency exit at every 500 metres, turning cavern at every 2.2 km, air quality monitoring at every 1 km, and broadcasting system and automatic incident detection system with CCTVs at every 250 metres.