Debris has been found near the Titanic by the US Coast Guard, in their search for the tourist submarine Titan which went missing on Sunday.

The Titan, a personal submarine built for OceanGate Expeditions to take tourists to view the HMS Titanic, began its descent on Sunday morning. Aboard were five people – British billionaire Hamish Harding, French veteran PH Nargeolet, Pakistani businessmen Shahzada Dawood and his son Suleman, and OceanGate CEO Stockton Rush. Only an hour and 45 minutes after the craft submerged, it ceased communication with its mothership, the Polar Prince. Those aboard searched for the submarine with limited resources for eight hours before calling in in the U.S. Coast Guard.

The submarine Titan carried enough air for the long journey over two miles down, plus an emergency supply of approximately four days. And it could not be opened from the inside, meaning that even if the craft managed to return itself to the surface, there was a very strict clock for finding and recovering it.

The search attracted more aid and more attention as it went on and that clock counted down, with some news networks posting a live countdown on their scrolling feeds. But nothing was found. Brief hope came when several of the rescue craft reported hearing knocking sounds via their sonar arrays, but none were able to track the sounds down, or prove that they came from the Titan.

But Thursday, only hours after the clock ran out, the US Coast Guard reported finding a ‘debris field’ via a small remote operated vehicle, one that doesn’t belong to the Titanic.

The most likely scenario is that the Titan, less than two hours after submerging, suffered a catastrophic failure and imploded under the pressure of the depths. In that case, the deaths of all five passengers would have been instant, a much more merciful fate than suffocating together in the world’s most expensive coffin, powerless and being rolled by currents along the sea floor in temperatures close to freezing.