Sunken ping.
Thanks colorado tanker. And wow. Those interior shots are spectacular. Some years ago an underwater explorer in Lake S found a lumber-hauler about the age of this ship. The cargo was intact and consisted of large logs including elm. He had planned to raise the logs as they are probably of great value. The last large (worth cutting) tree cut down around here is from, you guessed it, about 1910, the trunk cross-section was about the height of the men who'd cut it down.