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Radio telescopes are often built into sink holes or natural depressions in the landscape.* This is opposed to optical telescopes which are usually placed on mountains to avoid as much atmosphere as they can.

So why natural depressions? Is it easier to build them in a hole, less interference from local emissions, or something else?

*Or at least the well known ones like former Arecibo Observatory and Five-hundred-meter Aperture Spherical Telescope

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    $\begingroup$ You save the work of creating an artificial depression ;-). $\endgroup$
    – Peter - Reinstate Monica
    Aug 31 at 13:02

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Depressions are ideal for extremely large single-dish telescopes like Arecibo and FAST for several reasons, but the single greatest advantage is structural. These instruments are several hundred meters in diameter and therefore require delicate support systems to maintain their shape and avoid collapse. You could certainly try and build a freestanding dish, but it would be even harder than it already is. The karst depression FAST was built in, Dawodang, is close to spherical, like the telescope, making building the support structure comparatively easy (Zhu et al. 2018).

Besides the structural benefits unique for these large dishes, building a telescope in such a depression provides natural shielding from radio frequency interference. Some radio telescopes built on flat ground, like those at the Green Bank Observatory, are similarly protected by the surrounding hills (as well as, in Green Bank's case, a large radio quiet zone). Both Arecibo and FAST benefit from walls of their depressions -- one reason Arecibo's current site is considered promising for the telescope's possible successor, regardless of configuration (Anish Roshi et al. 2021).

Finally, depressions provide drainage. Water can flow to the center of the natural bowl and, particularly in karst topography, subsequently seep into the ground -- one advantage they have over other types of depressions, like craters or mine pits. While karst is permeable, it isn't fragile, so you don't have to sacrifice stability for the sake of drainage.

Basically, the structural advantages of depressions are applicable just to these extremely large telescopes, but there are side benefits, albeit ones which other, smaller radio telescopes not located in depressions can achieve in other ways.

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    $\begingroup$ Exactly the answer I hoped for, thank you for the citations as well. $\endgroup$
    – joeyfb
    Aug 29 at 16:34
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    $\begingroup$ @joeyfb You're welcome! Though I'd suggest waiting to accept this answer -- it might discourage other folks from answering as well, and they could add helpful information. $\endgroup$
    – HDE 226868 ♦
    Aug 29 at 16:36
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    $\begingroup$ "The karst depression FAST was built in, Dawodang, is close to spherical, like the telescope" So the "resting" shape is spherical, but it is deformed into a paraboloid when in use? $\endgroup$
    – Acccumulation
    Aug 30 at 2:08
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    $\begingroup$ @Acccumulation The off-axis aberration of a paraboloid is bad, so if you're going to steer the telescope by moving the feed around, a spherical shape is better. You can design the feed to compensate for the resulting spherical aberration. $\endgroup$
    – John Doty
    Aug 30 at 11:56
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    $\begingroup$ Because low EM frequencies aren't as sensitive to atmospheric disturbances, there's also less need to be high up on top of mountains compared to optical. So one can prioritize the structural benefits to save cost without hurting the science capabilities. $\endgroup$
    – Paul T.
    Aug 30 at 13:35

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