The section of tunnel is so consistently damp that BART maintenance crews call it “the rainforest.” As opposed to "the pricey real estate above," as BART writes, "BART’s tunnels in downtown San Francisco are below the water table," where "seawater or freshwater constantly seek to convert the section of trackway between Embarcadero and 16th Street/Mission Stations into a flume ride."
To keep that from happening, BART was originally constructed with steel plates bolted together over walls. Decades later, persistent water has made its way through caulking material, leaking through though the steel casing at the joints. That creates a number of problems — fractures in tracks caused by water damage, for example.
Draining that water further underground isn't possible, BART says — or, rather, it already does to an extent, but more drainage isn't an option: "Draining too much water from the whole downtown area could cause Market Street to sink (or possibly collapse)," they say. The most viable solution is a naturally occurring mud — bentonite. Environmentally friendly and even sometimes used as a medicine on the skin, implementing the substance into the tunnels won't be easy, or cheap, but hey, it beats taking a flume ride to work.
Actually, scratch that, a flume ride to work sounds sort of fun.