Reading about the two nacro subs captured in the last year it's pretty clear you don't even need to go deep.
For a long time I was under the impression that diesel subs stand out like a sore thumb on sat photos and nation states watch them all like hawks, but apparently not and drug cartels can just roll around as they please in them.
In many ways its harder to see something through water than it is to see it through rock. Ground-penetrating microwave radar can get through tens of meters of quartz but centimeters of water. VLF and ULF penetrate 10-100x farther through the ground than through seawater. The frequencies that can penetrate more than a few hundred meters of water are around the same as the ones powering your lights. Antennas at those frequencies are miles long. You need special, non-conductive soils and bedrock to make them work. In short it's a real pain in the butt.
After 200 meters the ocean is practically opaque. Objects much deeper than that are reflecting a handful of photons. Below 500-1000 meters you're talking about photons per second at the surface.
Water is one of a quite small number of general radiation absorbers. To block alpha/beta/gamma radiation you need pure density- more mass per volume to slow down high energy particles. You can use heavy atoms like lead or uranium, or very densely packed lighter atoms. Neutron radiation is different- the actual number of atoms per volume is critical to maximizing the number of scattering events. That means you use things like polymers- hydrocarbons, so you have as many small atoms (hydrogen) in a volume as possible. Water is one of the denser liquids, while still being 11% hydrogen by mass, vs 14% for pure polyethylene. That makes it quite good.
Even sound isn't great underwater, relatively speaking- being a fluid in motion, there's a constantly-changing distortion on everything. The thermal conductivity also means thermal signatures spread out quickly.
Water is a poor medium for photons, but a great one for sound. That's why the US and other countries surveil the seas with linked arrays of underwater microphones, i.e. the Integrated Undersea Surveillance System (IUSS).
Diesel subs at 30 meters become nearly invisible. They're hell to find. And if they stop, they're really, truly silent. The best way to find one is don't lose it in the first place. Track it from base it's entire trip.
Pardon my layman ignorance on this, but they still need to surface pretty often right? And the diesel engines leave a distinct signature when they surface to recharge the batteries?
Watched countries argue a bit over diesel vs nuclear submarines and this is where I've been imprinted with the idea that nuclear subs are the only actual true stealth subs you can build. I may be wrong here, but the evidence seems overwhelming based on usage. Diesel subs are for homeland defense purposes and don't worry about the enemy knowing their whereabouts, nuclear subs go silent for months and everyone is completely clueless as to where they actually are.
True. But they're stealth enough, can operate with a snorkel without completely surfacing and modern li-ion battery tech allows them to operate underwater for weeks.
Small ones. Additionally, the military intelligence agencies with access to spy satellites probably don't care about blowing their cover or capabilities discovering makeshift drug running submarines.
For a long time I was under the impression that diesel subs stand out like a sore thumb on sat photos and nation states watch them all like hawks, but apparently not and drug cartels can just roll around as they please in them.