I even submerged it in a pool and still can't find the leak (yes, that runs the risk of introducing water into the mat, but I'm desperate).

Anybody else had this trouble?

I can't get through a night without going nearly flat on it, so the hole or holes must be very small. I guess I inflated or deflated it over something very sharp.

Of course, I'm past the warranty period!

Try a spray bottle with soapy water and cover the whole bottem of the mat. It should start bubbling when you put some pressure on the mat.

This method worked great for an REI used sale thermarest.

your valve might be done. If there was a hole in the part where the down is contained, then down would be everywhere. Warranty it. Exped has 5 years.

"What is the warranty for Exped products?

Exped UL sleeping mats carry a 2 year limited warranty. All other products carry a 5 year warranty on defects in materials and workmanship. At our discretion, Exped will repair or replace any part that fails due to a manufacturing defect. Failures due to normal wear, negligence, and acts of nature can usually be repaired for a nominal charge."

I'll try the soapy water and check the valve, too.

I'm not sure that the downmat is a "UL sleeping mat." There is the new "Synmat UL 7," but "UL" does not appear in the name of the downmat that I have noticed.