You need to breed the slime. Here are the steps:
* Give the spoiled food to the slime near the shore of the small oasis (or, if you've already given the food, just pet it through the dialogue).
* Reach max milk.
* Sleep to trigger the slime encounter.
* Go through slime gestation and birth.
* Your new "child" will now appear in your inventory and can be used on the web door to access the underground area.
But underground area is still in development right now, I've just finished the model for the underground race to use, I’ll need some time before the next part is playable.