Running down the broken strairs of one of the houses in "Chapaevsk" may cause the character to fall through the ground eventually ending up in the usually unaccessible room to the left. This seems to happen 2 times of a total ammount of 4 tries.

Running in that room seems to make you fall through the floor sooner or later.
Only way out seems to be the ladder.

Simply run in the ladders direction and press "y" to lie down in a dolphin dive manner...
The ladder symbol is visible for a slit second. Pressing "F" might get you back on the ladder making you able to leave the building unharmed.