Tumblr user Glumshoe, if I understand their bio correctly, works in science communications on the Lake Michigan shore and thus knows something about how the mysterious and sometimes dangerous sand dunes in Indiana Dunes National Park work. It was in that location that, several years ago, a 6-year old boy sank into and almost died in the interior of this real-life sarlacc.
Physically speaking, how is this possible? Glumshoe drew this comic and explained:
The current leading geological theory as to how this happened is that the organic material you engulf, like trees, slowly decompose beneath your slopes, leaving behind unstable voids held together only by the fragile remains of the decayed material. When these voids are walked over, they collapse, forming sudden sinkholes that can swallow visitors whole. The rules that typically govern stationary dunes, or wandering dunes in areas that are not forested, no longer apply to you. You are unpredictable and dangerous and have remained closed to visitors except on guided hikes ever since.