Greetings, Are we talking bulb sensu stricto or anything tuberous. If true bulb, I second the Crinum bulbispermum suggestion - though I don't know how dense they are. Tim, did you weight the 20" dia Crinum you dug? I know Brunsvigia josephinae can push 40 lbs. If tubers are in the running, then I think a number of curcurbits are the champs far and away: Marah species have documented tubers over 400 lbs - and that was just the portion of the tuber that was extracted and weighed. I have unearthed the top of one that was easily as big as a recliner. -|<ipp