yo anon talking cemex and clinker transit ships may be on to sumthin.
cargo holds that lod from the top with a lip to prevent climbers. big enuff to drop a tractor into for cleaning (see 30sec vid) and drains to hose down human waste etc… also likely easy to pass inspectors with a false roof/bottom above stowaways so when opened it looks like something else.
https://youtu.be/lcToeH7tUEk
https://youtu.be/iQfaQ1I-KZY