Scientists have discovered a subterranean river 4km (do your own conversion) beneath the Amazon River that may be 6,000km long and hundreds of times wider than the Amazon.
From The Guardian:
Both the Amazon and Hamza flow from west to east and are around the same length, at 6,000km. But whereas the Amazon ranges from 1km to 100km in width, the Hamza ranges from 200km to 400km.
The underground river starts in the Acre region under the Andes and flows through the Solimões, Amazonas and Marajó basins before opening out directly into the depths of the Atlantic Ocean.
The Amazon flows much faster than the Hamza, however, draining a greater volume of water. Around 133,000m3 of water flow through the Amazon per second at speeds of up to 5 metres per second. The underground river’s flow rate has been estimated at around 3,900m3 per second and it barely inches along at less than a millimetre per hour