C++调用设计地下系统隐式删除默认构造函数

我真的不确定我为什么会犯这个错误。给出错误消息：

Line 132: Char 38: error: call to implicitly-deleted default constructor of 'UndergroundSystem'
        UndergroundSystem* obj = new UndergroundSystem();
                                     ^
Line 2: Char 56: note: default constructor of 'UndergroundSystem' is implicitly deleted because field 'getthem' has a deleted default constructor
    unordered_map, pair> getthem;
                                                       ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/unordered_map.h:141:7: note: explicitly defaulted function was implicitly deleted here
      unordered_map() = default;

class UndergroundSystem {
    unordered_map, pair> getthem;
    unordered_map> idcenter;
    
public:
     UndergroundSystem() {
    }
    
    void checkIn(int id, string stationName, int t) {
        idcenter[id]={stationName, t};
    }
    
    void checkOut(int id, string stationName, int t) {
        auto passenger=idcenter[id];
        string fromt=passenger.first;
        string whereto=stationName;
        int start=passenger.second;
        idcenter.erase(id);
        getthem[{fromt, whereto}].first++;
        getthem[{fromt, whereto}].second+=t-start;
    }
    
    double getAverageTime(string startStation, string endStation) {
        auto array=getthem[{startStation,endStation}];
        int size=array.first;
        int sum=array.second;
        return sum/size;
    }