Mike joined us Wednesday on "TMZ Live," and detailed his experience on the Titan ... the sub that went missing this week in the Atlantic Ocean while attempting the exact same journey he made last summer.

For Mike, his trip included a few bumps and some technical difficulties ... but he says he kept his cool inside the cramped sub, even getting some shut-eye on the 2.5-hour descent to the ocean floor.

He says they stumbled upon the wreck and only had about 30 minutes to check it out from the tiny viewport ... but he got his pictures.

Mike was able to keep calm even in the face of some challenging conditions ... as he tells us, it's pitch black 2.5 miles under the surface and the sub creeps along in the dark looking for the wreck, usually without much help from the navigators back on the surface.

Plus, there's the fact Mike says the waiver he signed before the expedition made it very clear death was a possible outcome.

It's interesting ... Mike says he was actually comforted by the relative simplicity of the sub, including the video game controller steering the Titan.

As you know ... the Titan went missing Sunday during an expedition to the Titanic and a search and rescue effort is underway ... the clock is ticking and time is running out.

Our conversation with Mike is super interesting and insightful ... and he tells us why he went down there in the first place, and under what circumstances he'd actually go back again.