The pomelo is the largest fruit
and can grow larger than 8 inches in diameter. It is pale green to yellow when ripe with white, pink or red flesh. The white flesh is most common, but in California recently all I've been able to find are pink varieties,
marketed as "red pummelo" and "pink pummelo," pummelo and pommelo being other names for the same fruit.
It is very similar to a grapefruit, but sweeter and more mild. The skin is spongy and much thicker than a grapefruit.
The segments inside are not as uniform as a grapefruit's and the membranes separating them are much thicker and much more difficult to remove.
They are also much less edible than the grapefruit membrane.
I found the membranes so tough that I sometimes had to resort to cutting them open with a knife before I was able to peel it off.
As 1001 Foods You Must Taste Before You Die puts it, the "segments are composed of myriad juicy 'beads'" which "burst satisfyingly in the mouth."
As with so much other good fruit, it is native to Southeast Asia. I ate two recently, one was very nice and juicy, the other kind of dried out and not very satisfying. I think I do prefer it to grapefruit, but it is more difficult to open up and eat.