What is the temperature 55 km beneath the surface of Mars?

If we look at Mars' possible geothermal gradient (see Earth's) which is about 25 °C per km. Using the low estimate of Mars's gradient to be 1/4 that of Earth's Source, that's a bit over 6° C per km. so 55 km, 330° C. Added that to Mars' average surface temperature of -55 C, you're talking 275° C or 527° F at 55 km underground, and that's a low estimate.

Estimates of the thickness of Mars' crust are often less than 55 km and if you're talking about digging into mantle or even, half way to the mantle, that's both an unsustainable dig and unlivable temperatures.

Source

The average thickness of the planet's crust is about 50 km (31 mi),
with a maximum thickness of 125 km (78 mi)

and

Source

Also, small point, but the 55km estimate assumes uniform temperature. The simpler pressure calculations don't take into account temperature fluctuation. With warmer temps and warmer lighter air at the bottom of the dig, which would be inevitable, you'd probably need 60 km, maybe 65 km of depth to achieve 1 atm. There's also circulation issues when you have cold heavier atmosphere above warm lighter atmosphere and there's drainage/flooding problems for the lowest part of a valley, so to avoid flooding, you might want to keep the surface temperature below freezing, to avoid runoff and silt build-up, or dig even deeper so there's a lake of sorts that catches the melting ground-ice around the dig and the living area is above the lake.

This doesn't mean that deep valley living on mars will never be possible, but it's likely impossible with it's current atmosphere. With extensive terraforming, tapping gases from underground and/or from it's ice-caps or by crashing comets into Mars (that would take a lot of comets), over time, enough of an atmosphere could be created where deep valley living might work. A simulated atmospheric pressure can also be created with a bubble-dome of sorts, or one of my favorites, a stretch-wrap around the entire planet, but for now that's way beyond our means.

For the near term, living underground on Mars is probably the way to go. Dig 50 or 100 feet underground and you'd have the means to create a near vacuum seal, which on Mars, you'd want to do, you could create earth like air pressure and you'd be protected from solar and cosmic rays and access to plenty of underground water. underground on Mars is likely the way to go, at least, in the near future. There's no shortages of articles on this. Here's one 6.

Humans can also live at less than 1 atm. People have settled as high as 16,700 feet, which is somewhere in the neighborhood of 55% of 1 atm.