The Mayan settled in present-day Belize, around 1800 BC - 250 AD. So far, scientists have no idea how they disappeared, but, there are theories.
I think their best accomplishment is their aquaducts because they can get fresh water from them. Also, it could attract some food sources, like wild pigs, or monkeys.
Another Accomplishment is the Mayans invented rubber more durable, bouncier, and thicker before the Europeans did, and the Mayans were WAY ahead then them, too.
What I Think
I think their weakest accomplishment is their hand writing, because there is nothing protective, and it takes a long time to teach someone how to write in that type of writing. But, It does give us more info. about them.
Some Theories on how they disappeared are from warfare, disease, famine, or a disastrous environment change.
Honestly, I Think its warfare, because if they were fighting the same enemy, they can learn their techniques, and plan ahead.