The continents have drifted vast distances since the Quaternary. A massive rift of land which broke away from Africa along the East African Rift has moved across what was the Indian Ocean and collided with the southernmost corner of Southeast Asia. As the two landmasses came together, a vast inland sea was created between in the area that was once the Bay of Bengal. The massive forces created by the colliding tectonic plates buckled the landmass, giving rise to a volcanic mountain range along the line of fusion. Over time, the inland sea became almost entirely cut off from the oceans to the south.
Water runoff from the mountains formed rivers which washed fertile sediment into the landlocked sea. Eroded material from the newly-exposed rocks was carried downwards, filling the basin and making it shallower and rich in nutrients. Gradually, the inland sea diminished, freshwater from the mountains mixed with the residual saltwater, and a vast, brackish swamp was formed.
100 million AD, the Bengal Swamp covers hundreds of thousands of square miles. Sediment carried by slow-moving channels and meandering rivers makes the water thick and impenetrable to light. Sedimentary deposits have a series of oxbow lakes and backwaters separated by muddy islands and flats. The Bengal Swamp is comparable in appearance to the great lowland coal swamps of the Carboniferous period, 358.9-298.9 million BC.
The climate of the Bengal Swamp is hot. Its proximity to the Equator and the shelter provided by the surrounding mountains mean that average temperatures are about over 100 degrees Fahrenheit (40 degrees Celsius). Water is plentiful, running down from the mountains in an intricate network of rivers. Humidity is extremely high, averaging 99 percent all year round. The muds and soils are constantly replenished by nutrient-rich volcanic ash.
A greenhouse environment like this is an ideal place for vegetation to grow. Plant life chokes the waterways and spreads across the lakes. Thickets of tropical plants clothe the sandbanks and deltas. Tightly spaced trees stand where any land is solid enough to hold them, spreading deep canopies of branches and leaves overhead and stabilizing the mud with their network of roots.
A host of dangerous creatures dwell in the murky backwaters and shallows of the Bengal Swamp, beneath the tangle of thick, choking vegetation.
The Bengal Swamp has become so dangerous, so full of large swimming predators, that some other water creatures have taken refuge out of the water. Indeed, the atmosphere of this hot swamp is so humid that many aquatic animals can spend time on land and not suffer any discomfort.
In an environment such as the Bengal Swamp, where vegetation flourishes, the presence of large herbivores is no surprise.