Background: Peatlands are becoming more vulnerable to smouldering fires, driven by climate change and human activities.

Aims: This work explores the persistent burning, propagation, and emission of the deep peat fire.

Methods: Laboratory experiments are conducted with a 1-m deep peat column, and smouldering fires are initiated at different depths.

Key results: We found localised burning and multi-directional smouldering fire spread in deep peat layers. The smouldering temperature first decreases with depths up to −40 cm (from around 550 to 350°C) and then remains at about 300°C in the deeper layers. High moisture content can slow down in-depth fire propagation and reduce the burning duration.

Conclusions: Peat fire can burn in deep layers for weeks, and its combustion is incomplete with small mass loss, because of a limited oxygen supply and low smouldering temperature. Measuring the carbon monoxide concentration near the surface can detect underground fire and monitor its intensity.

Implications: This work helps reveal the underlying mechanism of the in-depth smouldering wildfires in peatland and supports future larger-scale peat fire experiments in the field.