Some 60 holes drilled for hydrocarbon exploration in the Mackenzie valley afford the opportunity to obtain precise measurements of deep permafrost thicknesses (depth to 0°C) and ground temperatures, and are presented here on maps and graphs. Permafrost thickness is less than 100 m in the Mackenzie Delta, and up to 663 m in the adjoining Tuktoyaktuk coastlands. Permafrost varies from 35 m to 143 m in the Norman Wells area, but is thin or absent in the Fort Simpson and Yellowknife areas. Variations in ground temperatures with depth reflect differing thermal properties of soil and rock, proximity to lakes or rivers, and residual thermal disturbance due to drilling. In some cases, subsurface temperatures show features that are relict from several thousand years ago when the local site environment was different from today, due for example, to more widespread lakes, changing shorelines, or river channels.