Einstein's general ‘Theory of Relativity’ nearly a century ago describes how space-time is affected by mass. The associated fluctuations in the space-time fabric of the Universe due to gravitational effects are called gravitational waves (GWs). These distortions spread out like ripples from their origin in violent events believed to be associated with certain major binary systems, such as two black holes colliding. GWs are quite rare and weak (low-amplitude), yet their study is expected to provide important insight into the nature of the Universe. They have never been directly detected although indirect effects on other planetary systems have been recorded, making them a sort of scientific Holy Grail. Scientists initiated the EU-funded project 'Einstein gravitational-wave telescope' (ET) to develop the design concepts for a third-generation detector with sensitivity more than 10 times that of the latest second-generation devices being built. Work focused as well on the necessary infrastructure to minimise interference as the detectors may be updated and replaced over time but the infrastructure must be able to host them all well. To improve detector design and sensitivity, noise issues particularly at low frequency needed to be dealt with. Second-generation detectors were affected by seismic noise, Newtonian noise (produced by gravity gradients or local fluctuations in the Earth's gravitational field) and thermal noise associated with the detector setup. To limit seismic noise and improve sensitivities at low frequencies, the observatory design included an underground site for the detector. The infrastructure will also include cryogenic facilities to cool the mirrors and limit thermal vibration of test masses. A rough cost analysis is included to evaluate the financial feasibility of the project. ET project heralds an era of GW detectors that can detect elusive GW waves within months of data collection, unravelling the mysteries of the cosmos. From the birth and death of stars to the nature of the dark matter, ET could change our understanding of the Universe significantly.