Lake Neusiedl (Neusiedler See) is in the easternmost part of Austria, with a small part of it being located in Hungary. It's inscribed on the UNESCO World Heritage list.
Lake Neusiedl in the winter
Neusiedl Lake can be reached by train from Vienna or by bus from Eisenstadt and Sopron.
In summer Neusiedl Lake is known for winter sports as well as swimming.