This strange object was built on the coast of Great Britain. What is it?

sun clock
storm detector
acoustic mirror
emergency radio receiver
Prior to World War II and the in­ven­tion of radar, acoustic mirrors were built as early warning devices around the coasts of Great Britain, with the aim of de­tect­ing airborne in­va­sion­s. The de­vel­op­ment of radar put an end to further ex­per­i­men­ta­tion with the tech­nique.

Answers distribution

sun clock
storm detector
acoustic mirror
emergency radio receiver
answers
flagFinland
0%
0%
100%
0%
47
flagItaly
0%
1%
98%
0%
53
flagVietnam
0%
0%
92%
7%
13
flagBulgaria
8%
0%
91%
0%
12
flagCzech Republic
5%
5%
88%
0%
17
flagGreece
6%
0%
87%
6%
16
flagSpain
0%
9%
86%
4%
22
flagthe United Arab Emirates
11%
0%
84%
3%
26
flagGermany
9%
3%
76%
9%
172
flagthe Netherlands
11%
8%
74%
5%
35
flagPoland
10%
6%
73%
9%
10462
flagRepublic of Ireland
11%
7%
73%
7%
26
flagNorway
18%
4%
72%
4%
22
flagUnknown
14%
3%
68%
12%
57
flagthe United Kingdom
15%
6%
67%
9%
450
flagthe United States
12%
5%
67%
14%
225
flagCanada
7%
0%
61%
30%
13
flagLithuania
0%
14%
57%
28%
14
flagFrance
22%
0%
55%
22%
18
flagIndia
27%
19%
27%
24%
62

Gender

men79%
women75%
sun clock
6%
9%
storm detector
5%
5%
acoustic mirror
79%
75%
emergency radio receiver
8%
8%

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This strange object was built on the coast of Great Britain. What is it?
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