Muddy swamps (sometimes referred to as a BOG) are a dungeon feature added to UnNetHack from Nethack Brass.
For most purposes (trying to engrave, gremlin multiplication, damage to iron golems...) the muddy swamp works just like water.
You can't drown in a bog, but you can get "stuck in the mud".
Bogs are most commonly encountered within the swamp special room.
Frequently encountered in the Dragon Caves.
One particularly nasty aspect of getting stuck in a muddy swamp is that you have a 20% chance of your inventory getting wet (trap.c, line 4350)!