Dhaka's one of the most popular historical site is Lalbagh fort. However, this 16th century brick-built Mughal fort, established to protect Dhakaites from the invasion of Portuguese and Arakanese pirates, possesses some of the strangest historical secrets. The most famous of them is the mystery regarding its secret passages. There are several underground passages in the fort which are now sealed at present. However legends say that two of these underground passages led to now ruined Zinzira Fort which was situated on the other side of the Buriganga River. The other passages were built as mazes where invaders and intruders would lose their way and finally be starved and suffocated to death. In fact, it has been claimed that during Sepoy Revolution against the colonial rulers in 1857, many of the surrounded and defeated sepoys tried to flee through those passages and lost their lives inside those passages. Even British soldiers who tried to chase them and arrest them also never returned. Later British researchers sent an elephant and dogs to those passages but they never came back. Then those passages were sealed permanently. Still the mysteries behind these secret passages remain unsolved and the actual purpose of these architectural mazes remains unknown.
Sources: Banglapedia: National Encyclopedia of Bangladesh