What is this durian-like fruit?

Rachel and I found this at Fort Canning Hill.

Out of curiosity, can anyone tell us what fruit this is?

Strange looking fruit

In the tree

There were many of these fruits lying around on the grass patch while some were still attached to their stalks. We found them interesting due to their resemblance to durians which Rachel is crazy about. Initially, she thought she found durian trees and was very excited about it.

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