Worlds second largest hole is found in Bay of Bengal :
Swatch of no Ground a trough-shaped marine valley or canyon that cross the continental shelf diagonally and situated on the south of the Ganges-Brahmaputra delta. It is also known as Ganga Trough. similar delta-front troughs are found off the mouth of the Indus River known as Indus Trough and off the West Side of the Mississippi delta, the Mississippi Trough. Swatch of no Ground has a comparatively flat floor 5 to 7 km wide and walls of about 12′ inclination. At the edge of the shelf, depths in the trough are about 1,200m. It has been suggested that the Swatch of no Ground has a seaward continuation for almost 2,000 km down the bay of bengal in the form of fan valleys with levees. The sandbars and ridges near the mouth of the Ganga-Brahmaputra delta pointing toward the Swatch of no Ground indicate that sediments are tunnelled through this trough into the deeper part of the Bay of Bengal. Slumps, growth faults and evidence of mass movement coupled with high sedimentation rates near the Swatch of no Ground provide dramatic evidence that modern sediment is being channelled off-shelf through the submarine canyon to the Bengal Fan. Studies on the bengal deep sea fansuggest that Swatch of no Ground is feeding the Bengal Fan by turbidity currents. Most of the sediment of the Bengal Deep Sea Fan has been derived from the confluent of ganges and Brahmaputra rivers, which drain the south and north slopes of the Himalayan, respectively. Under the present condition perhaps low-density turbidity currents and sand cascading are perhaps dominating process of sediment transport from the shelf to the deep sea through the Swatch of no Ground.
There is some controversy regarding the origin of Swatch of no Ground. However, it is generally believed that during the Pleistocene (2-0.1million years ago) lower sea level, Ganga-Brahmaputra River was discharging its sediment load directly on the shelf edge. Combination effect of river flow and turbidity currents generated at the shelf break and upper slope was responsible for the formation of the Swatch of no Ground. Evidence from the Bengal Deep-Sea Fan also tends to support this view. It has been observed that turbidity currents during the Pleistocene lower sea level dominated sedimentation on the Bengal Deep-Sea Fan and the sediments were distributed on the fan by a system submarine channel originating from the Swatch of no Ground.
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