Atmospheric pressure increases the deeper you go & the highest growing plants on Earth have been found at a height of 6 km so how deep a trench would we need on Mars to provide similar air pressure? World’s highest plants discovered growing 6km above sea level
Mars' atmosphere scale height is, depending on who you ask, 10.8 to 11.1 km.
- Pressure at the bottom of Hellas Planitia: 1.16 kPa
- Earth sea level: 101.3 kPa
- Earth 6km altitude: ~50 kPa.
So we need air pressure to increase by a factor of about 43; natural log of 43 = 3.76 scale heights -- so we need a trench about 41km deep. Start digging!
This gets equivalent air pressure, but there's almost no oxygen. Partial pressure of CO2 on the other hand is about 2400 times higher -- Mars's surface level atmosphere offers more CO2 than Earth as it is.