It turns out that, when zoomed out, the underlying maps Strava is using are quite out-of-date (at close zoom levels, the maps are accurate). This leads to some very interesting contrasts. These older maps show, for example, the current airport island of Yeongjongdo (영종도) in its per-consolidated form, before the multiple former islands were filled-in to create one large island. Yet, of course, the heatmap signatures from joggers and cyclists, who apparently circumscribe the island as part of their exercises routines, are overlayed on top. This unintentionally results in a stunningly beautiful way of looking backward in time, and comparing the former geography with the current boundaries.
Here, for example, you can make-out the current island's features to the left. At the top right, the now well-built-up Gyeongin Ara Waterway canal (경인아라뱃길) is visible against the backdrop of former ocean, the banks of which I can vouch for as being a lovely cycling/jogging course.
|Strava heatmap of Yeongjongdo|
The Korean-language side of Wikipedia shows this nice illustration showing the original layout of the islands (grey), where the landfill was placed (yellow), and the extent of the current airport facilities (green):
User:Mer du Japon - 기존 지도를 모사하여 자작의 "영종용유삼목신불도". 위키백과에 의해 퍼블릭 도메인으로 라이선스됨.
An additional similar side-effect is that these maps do not include any of the development of Songdo New City, which is also a popular area for cycling and jogging. Here too, you can see the outlines of what was once sea and is now a city:
|Strava heatmap of Songdo area|
Strava's world map includes the entirety of Korea, so I encourage you to pleasurably browse around it. Feel free to add any similar cool locations in the comments. Remember that, while also cool to browse up-close, you'll need to zoom out sufficiently in order to see these older background maps.
Strava Global Heatmap (focused on Incheon area)