Check out these photos, obtained by TMZ, that show the patch of ground that caved in as the "16 and Pregnant" star was driving his ATV, causing the off-road vehicle to pin him underneath ... crushing his skull in the process and taking his life before help arrived.

As you can see, there's a mini-landslide on a slope of a hill next to a dirt road ... this is where Sean's mother, Mary Hobbs, says he was parking the ATV to make room for her car when the accident happened. Now, there's a makeshift memorial where Sean died ... with flowers and signs that say, "We Love You Squishy." We're told that was Sean's nickname.

Mary tells us they're planning a permanent memorial here ... which is currently in the works.

His stepdad, Dan Wolfe, is building the memorial -- and he tells TMZ ... there will be a stone wall with a laser engraving of Sean's picture, plus a custom urn made from black cherry wood, laser engraved with Sean's thumbprint, picture, birth date and death date. A garden will also surround the memorial when it's all said and done.

Mary says her son grew up riding ATVs, dirt bikes, four-wheelers, etc. ... and he wasn't doing anything reckless when he died ... it was a freak thing, and an absolute tragedy.

Of course, Sean was on Season 6 of the show with his ex, Selena Gutierrez ... and now -- their two young children will grow up without their father, something Selena is agonizing over.

Selena tells TMZ ... their two daughters, 3-year-old Dareli and 19-month-old Esmi, are too young to understand what happened to their father at the moment ... but she's glad they got to spend time with him a few weeks before he died.

TMZ also obtained some family photos and videos of his last visit with his children ... showing Sean lovingly interacting with his kids. Selena says he was "an amazing father who loved his girls" and his mom says Sean's daughters were his whole world.

Sean was only 20.