I don't think this anime was posted before (or very long time). There is an antlion quicksand type in the anime "the third" with in bonus some tentacles
Too bad there is not a lot sinking view personally but why not ?
The scene is in episode 14 and begin aroud 10:00.
I can't find it in youtube so i don't know if this link will work for you (french sub)
http://www.adkami.com/index.php?page=manga&id=995&episode=014&version=2&genre=1