Quagmire is an extremely dangerous block likely to never release whoever falls into it. It uses the mechanics of spider webs while still being solid like a block, thus making you suffocate when you sink in.
This block looks like wet dirt. Check the category page for more types of dirt.
It can be found in marshy biomes (e.g. Dead Marshes, Midgewater, Swanfleet, Shire Marshes etc.), and is usually located near the water sources. It is possible to break the blocks with a shovel. If you don't have a shovel, you can only try to slowly move in the direction of nearby water and swim out of it.
Many NPCs can be found stuck in these quagmires. A player can lure opposing players/NPCs into a quagmire for an easy kill.
Because of the danger of suffocation, it is advisable to always carry a good shovel or lots of blocks(dirt, stone) when you are travelling in biomes that are known to have quagmire.
Quagmire was mentioned several times in the Fellowship of the Ring when certain characters are traveling through marshy areas of Middle-earth.
- If riding a horse and you get stuck in underwater quagmire, you can jump to escape from it.