US Nuclear sub has hit an unknown object in the South China sea

[tartare]

Hmm - a searchlight.
Noting that you're joking (?) as someone who knows very little about submarine warfare, I assume that any emitted radiation - even frequency hopping - of any strength and spectra is probably pretty good at broadcasting one's position, visible light, LIDAR, same issue, and infrared.
If `seeing' ahead underwater without giving away one's position was possible, shirley someone would have done so by now...?
Recc - the passive fathometer stuff is intriguing.
Knowing a little about passive radar in the aviation world, I presume at the depths most submarines are at, there is not enough biologic or ambient noise to provide any sort of passive `image' of what is ahead.
Advice from those who know more appreciated.
Interested to read too about how out of date some charts still are.
I had assumed that areas of interest such as the South China Sea were mapped and know to the millimetre - but clearly not so!
This whole business of underwater navigation is fascinating stuff.