Seismic soundings of Lake Vostok have been performed by the Polar Marine Geological Research
Expedition in collaboration with the Russian Antarctic Expedition since the early 1990s. The seismograms recorded
show at least two relatively closely spaced reflections associated with the lake bottom. These were initially interpreted
as boundaries of a layer of unconsolidated sediments at the bottom of the lake. A more recent interpretation suggests
that the observed reflections are side echoes from the rough lake bottom, and that there are no unconsolidated sediments
at the bottom of the lake. The major goal of this paper is to reveal the nature of those reflections by testing three
hypotheses of their origin. The results show that some of the reflections, but not all of them, are consistent with the
hypothesis of a non-flat lake bottom along the source-receiver line (2D case). The reflections were also evaluated as
side echoes from an adjacent sloping interface, but these tests implied unreasonably steep slopes (at least 8 degrees) at
the lake bottom. The hypothesis that is the most compatible with seismic data is the presence of a widespread layer of
unconsolidated sediments at the bottom of Lake Vostok. The modeling suggests the presence of a two hundred meter
thick sedimentary layer with a seismic velocity of 1700 -1900 m/sec in the southern and middle parts of the lake. The
sedimentary layer thickens in the northern basin to ~350 m
