What could exist buried in the earth that is sugary and can be used to sweeten food? It should be relatively common, and at a depth where it is only accessible to peoples specialized for digging (specifically, the people described in this question). Also, the sweet thing should be something that might have come to be on Earth
$\begingroup$ @ARogueAnt. I mean a species of humanoids with physical adaptations for digging $\endgroup$– Ichthys KingJul 24, 2021 at 7:45
1$\begingroup$ Liquorice? Not to mention that a large part of the sugar we all love is made of sugar beet roots; only France alone produces 40 million tonnes of sweet sugar beet roots per year. Worldwide production exceeds 250 million tonnes per year. And yes, we dig them out of the ground. Asking about a common crop grown worldwide as if it was something fantastic earns a downvote. $\endgroup$– AlexPJul 24, 2021 at 10:47
1$\begingroup$ @AlexP The fact that sugar beets are so widely cultivated seems to me like it would imply that it is easily extracted by humans, which conflicts with the question's statement that it should grow too deep for human access $\endgroup$– Ichthys KingJul 24, 2021 at 11:36
1$\begingroup$ Nothing, except your handwavium. Reason is simple, really, sun is on top so as air and production of sugar(all kinds of it) require energy and carbon $\endgroup$– MolbOrgJul 24, 2021 at 13:10
$\begingroup$ 5 answers and not a single upvote. And the usual culprits. You all know better! Peoples of the stack: if it is good enough to answer, it is good enough to upvote. Get King some rep! $\endgroup$– WillkJul 24, 2021 at 19:40
Manna (but not from above).
During the wintertime, tree-sap sinks beneath the ground into the taproots of the trees (the ones that sprout from the main side-roots and then go straight downwards). These are only accessible beneath the canopy of the surface roots, and a dig must start some distance from the tree-trunk and then curve inwards underground.
The Ash tree (AKA. spruce, or fraxinus) produces in the summer, a sweet, protein and mineral rich sap, which during winter months can be drained from below. Cutting the tip off a tap-root, will enable several liters (from a medium/large tree) to be drained without killing the tree itself as the many other tap roots are enough to get the thing sprouting properly in spring.
There's even one gentleman who does it for a living and to keep an ancient tradition alive (admittedly he takes it during the sap-up months, from the trunk though).
Aside from being toxic, lead is also very sweet to the taste. It has historically been used as a sweetener with predictable consequences.
Lead on it's own is naturally occuring and sweet, but with a little bit of simple chemistry lead acetate can be made, which is a sweet salt soluble in water and glycerin.
Ideal for sweetening desserts and wines.
A lot of tubers grow underground, and some need some digging to be reached, like yam
Yams in West Africa are typically harvested by hand using sticks, spades, or diggers. Wood-based tools are preferred to metallic tools as they are less likely to damage the fragile tubers; however, wood tools need frequent replacement. Yam harvesting is labor-intensive and physically demanding. Tuber harvesting involves standing, bending, squatting, and sometimes sitting on the ground depending on the size of mound, size of tuber, or depth of tuber penetration. Care must be taken to avoid damage to the tuber, because damaged tubers do not store well and spoil rapidly. Some farmers use staking and mixed cropping, a practice that complicates harvesting in some cases.
In forested areas, tubers grow in areas where other tree roots are present. Harvesting the tuber then involves the additional step of freeing them from other roots. This often causes tuber damage.
Generally speaking tubers are rich in carbohydrates, because they are the energy storage of the plant producing them, therefore in principle all of them are either sweet or have the potential to be sweet. Something like a sweet potato.
So, you are looking for something which is an interbreed of a sweet potato for the sweetness and yam for the challenging harvest.
If you want the ultimate sugar rush then maybe grow sugar beet wikipedia link.
Honey from ground bees.
It's harvested by humans already
$\begingroup$ Good basis for an answer, could you give us a few more details? $\endgroup$ Jul 24, 2021 at 13:12