Final Answer:
The underground river that flows below the Amazon, featuring tidal floods called Pororoca, and forms a large delta at its mouth is the Rio Negro.
Step-by-step explanation:
The Rio Negro is a significant tributary of the Amazon River, flowing beneath the Amazon for a stretch. It is known for the natural phenomenon called Pororoca, a tidal bore where the river's tidal waters meet the ocean's waves, creating powerful surges. The Amazon River, after the confluence with the Rio Negro, forms an extensive delta at its mouth. The Rio Negro contributes to the vast water network of the Amazon basin, adding to the overall complexity and ecological diversity of the region.