A massive operation has been on since last night to rescue some 39 people believed to be trapped inside a tunnel after a glacier burst in Uttarakhand on Sunday caused flash floods that left 18 dead and over 200 missing. The 12-foot high and 15-foot wide tunnel at Tapovan in Chamoli was blocked with slush and debris, trapping workers of an NTPC thermal power plant that was washed away. The tunnel is over 2.5 km long with just one entry and bifurcates into two, officials said, adding that it was hard to tell where the workers were trapped or whether they were together at all.