A kūlgrinda is a type of Lithuanian underwater road or path, in which the pathway is completely under the surface of a swamp or bog. As you might expect, these constructions were primarily used for sneaky travels and emergency escapes.
Most of these paths were probably originally constructed in the 1400s to help protect local villages from the invading Christians, although estimates put some constructions as early as the 800s. The standard method of construction of large kūlgrindas was to place rocks, gravel, and other materials on top of frozen waterways in the winter, allowing them to sink gently into place when the ice thawed. This process was repeated as necessary, and the resulting pathway was leveled and perhaps reinforced with timber posts to prevent it from washing away.
Some of these were large enough to allow carriages to follow them (up to 8 meters in width), while others were essentially footpaths. Today the remaining kūlgrindas are knee-to-waist deep underwater -- and, in most cases, under a good layer of mud -- making it unclear how many of these were passable by carriage. It may be that most of these were once finished with a layer of oak logs, making them nearer to the surface and easier to navigate. However they were traversed, they were impressive constructions, spanning hundreds of meters of swampland, and passing through swamps as much as 7 meters deep. Stepping off the path, particularly in heavy armor, could well prove fatal.
Altogether, over 25 kulgrindas have been found in Lithuania, about half of them in Samogitia, and remains of others have been found in Kaliningrad Oblast, Belarus, and Latvia.
The word kūlgrinda comes from the Samogitian dialect of Lithuanian, from the words kūlis, meaning 'stone', and grinda, meaning 'pavement'. A kūlgrinda made primarily from logs is known as medgrinda, and may predate the stone constructions.