## 2 Structure of Present and Future Detectors
### The Proposed Seismic Attenuation Structures  
The two taller structures in the lower panel of Figure 1 are stacks of excavations (alcoves) connected by a vertical borehole. Each alcove harbors one of the vertical attenuation filters separated by pendulums longer than 10 m. They provide isolation for the most demanding cryogenic test masses of the low-frequency interferometers. The dotted side excavations near the base of these stack (dark blue in the 3D insert) house the noisy cryogenic chillers. Separate excavations are needed to isolate the vibration noise caused by the more than 150 kW of cryo-chiller power needed to cool the test masses and long vibration-isolated cryogenic shroud and baffles. These baffles extend for more than 50 m and have two requirements. They are needed to intercept all possible trajectories of water molecules that may otherwise reach and deposit on the mirror surfaces and must be vibration isolated to neutralize the effects of residual scattered light. The cooling power is delivered at an appropriate height along the seismic attenuation chains, with an appropriate separation from the test masses.  
Any other noisy equipment can be housed in similar alcoves. The access tunnels can be configured to reroute most of the main tunnel ventilation away from the test masses and recombination optics, where turbulence may cause atmospheric Newtonian and acoustic noise.  
The five intermediate-length towers, also shown in Figure 1, are for the room-temperature interferometer test masses and other main optical elements. They may be of different kinds and heights, depending on specific requirements. The three smallest structures contain seismic isolation for optical benches and other less demanding optics. An important point is that all seismic attenuation chains can be anchored to the rock above to avoid encroachmentof support structures around the optical elements. The inverted pendulum and top filter at the head of the long chains sit directly on the rock floor in the top alcove.  
It is important to note that alcoves with narrow and sealable access tunnels proved very effective to isolate from ambient noise or vibrations induced by ventilation or other machinery. When installing seismometers in the Homestake mine [24], it was observed that the instruments installed in similar alcoves had the lowest noise floor.  
The elimination of structural encroachment around the test masses is especially valuable in the locations where the beams of the high-frequency interferometer peel off from the main tunnel and to make space for the cryostats. The test masses can be simply housed along the main tunnels with little or no local tunnel enlargement.  
Other advantages provided by the proposed configuration are detailed in the following sections.  
We stress that the sketch of Figure 1 is only conceptual, and reasonable dimensions are suggested but not optimized. The exact topology, height, load, or size of attenuation stages for different optical elements are not yet determined. The vibration isolation solutions outlined can be scaled to meet the different requirements of various optical elements, while easy access to all attenuation chain elements can be maintained. Similarly, no effort is made here to decide excavation shapes and sizes, or to decide spacing along the beams to eliminate interference between different detectors. This will be the object of a complex tradeoff study involving mining engineers and physicists, with the aim to minimize costs, maintain safety and satisfy all scientific requirements. A rough cost comparison between the ET baseline design and the one proposed here was made [25]. The larger excavation volume of the first balance the greater complexity of the second. Within the evaluation error margin, no significant cost difference was found.