The benefits of reducing the thermal effect of global warming and structural damage by severe storms have renewed interest and drives to heavyweight constructions. One type of heavyweight construction using earth-sheltered construction known as an ‘Earthship’ has been developed and pioneered by US architect Michael Reynolds and recently introduced to the UK. The ‘Earthship’ takes on the concept of an ‘independent vessel’, which operates on a self-sufficient basis and is constructed largely from recycled and reclaimed materials. Although there are up to 3000 globally, only two Earthships have been built in the UK – one in Brighton and one in Scotland. The publicity resulting from the planning approval granted in April 2007 for the construction of sixteen domestic Earthships at the seafront of Brighton have raised public awareness to this type of unconventional construction. A key feature of the Earthship design is the use of a thermal store, which acts as a seasonal heat buffer to absorb heat in the summer and to release heat in the winter, to regulate the room temperature. This paper reports on the initial findings of a long-term study on the thermal behaviour of the thermal store and its effect to the indoor environment. It introduces the design and construction principles of the Earthship and outlines the process and equipment used in the thermal monitoring of the building. Indicative results from the analyses of the data collected at the early stage are reported and discussed. Initial findings have demonstrated the effectiveness of the thermal charging and discharging of the earth-rammed thermal mass, which appeared to moderate the extreme external temperatures.